I have a DW622 (basically a DW621 with the motor uprated to 1400 watts and a 1/2in collet as opposed to the 8mm max of your DW621), so what do you want to do?
Fundamentally the DW620 (single speed) and DW621 (variable speed) routers were originally introduced bu Elu as the Of1 and OF1e. They didn't have those names for long as Elu very quickly remarketed them as the OF97 and OF97e (and in due course after B&D took over they were also sold as the deWalt DW620/DW621 with the colour changed to yellow/black). This renaming was possibly to give some association to the MOF96 and MOF96e models which in many ways they were designed to replace (but which they never did!). As a result they share a lot of dimensions with the MOF96 - for example the same fence rods (8mm diameter) at the same centres are employed, which means that anything made to work on the fence rods of a MOF96 (or DW613/DW614/DW615 or Trend T5) will also ork on the DW621. That includes:
deWalt DE6905 Single bar trammel
Trend BEAM/001 Beam trammel
as well as many other devices such as the
Trend CRB . The collets and nuts are identical to those used on the MOF96, i.e:
Elu EZ49105 / deWalt DE6951 1/4 inch collet/nut set
Elu EZ49106 / deWalt DE6952 8mm collet/nut set
(in Europe there is also a 6mm model, but TBH that isn't relevant in the UK - 8mm is the biggest collet available for the DW620/DW621)
There are some areas where the DW620/DW621/DW622 are on their own, though. The
guide bushes used are almost unique to them AFAIK (only the Makita RP1110C uses the same style guide bushes) and the available range is limited (although there is still a greater range than you get with, say, Makitas or Hitachis:
deWalt DE6910 Guide bush, 10.8mm
deWalt DE6914 Guide bush, 14mm
deWalt DE6916 Guide bush, 16mm
deWalt DE6917 Guide bush, 17mm
deWalt DE6920 Guide bush, 20mm
deWalt DE6924 Guide bush, 24mm
deWalt DE6927 Guide bush, 27mm
deWalt DE6930 Guide bush, 30mm
deWalt DE6940 Guide bush, 40mm
Of those the most useful (I find) are the 16 and 30mm ones (used with a lot of ready-made jigs) as well as the 24mm (use with a 20mm cutter and a Trend hinge jig to handle radius corner hinges which are generally 10mm radius).
BTW there is a guy on eBay who sells an adaptor to allow them to be used with the (cheaper) MOF96-type guide bushes (3D Engineering - down in Devon or Cornwall). If you are a Porter-Cable guide bush user, then the Leigh 706R adaptor will allow you to use P-C compatible guide bushes with a DW621, etc
Another unique accessory is the fine depth adjuster which is very handy (
and highly recommended) when you need to make absolutely accurate adjustments to depth (e.g. for recessing for inset door ironmongery, use in a router table, etc)
deWalt DE6956 Fine depth adjuster
and while we are on the subject of router tables Elu and later DW did once make a really nifty mini router table (for the MOF96 and DW613/DW614/DW615) which also works with the DW620/DW621/DW622:
deWalt DE6900 Router table
The DW620, etc have far, far better dust extraction than most routers on the market (providing you have the bits, you do) and standard industrial vacuums from DW, Bosch, Metabo, Festool, etc all plug straight in.
Personally I think they are good routers with perhaps one caveat - the base is a bit on the small side for edge routing and the router really could benefit from a (home-made) acrylic offset sub base in something like 6mm acrylic.
As to when various router accessories should be used - the topic is far too big to answer in a thread. You really need to buy a book on routing and study that I feel, then enquire here when you have more specific questions
The DW670 is actually specifically a laminate trimmer rather than a router. At isn't the best laminate trimmer I've used, but it's not bad. Unlike the DW621, the DW670 family of trimmers were replaced about 6 or 7 years back and most of the accessories made for it are no longer available. It is the DW version of the Elu KF56 trimmer, and like the Elu when new it would have been supplied with the following:
DW6705 Trim base (the standard fixed base which you have)
Acrylic chip deflector (seems to be present)
Bearing edge follower (missing?)
Dust hood/rough trimming fence for lipping (present)
The standard base has an opening which works with the P-C guide bushes I mentioned above (want a metric set? Look at
Axminster Power Tools although Trend also sell 16, 17 and 30mm P-C compatible guide bushes or USA as they call them)
There were other bases available for the motor, namely:
DW6706 Tilting base
DW6707 Offset base
DW6708 Seaming base
DW6073 Oversize round sub-base
The first three are rather specialised bases and the DW6707 and DW6708 are probably only of use to anyone manufacturing laminate countertops, vanity units, etc because they are laminate-specific pieces of kit (not even sure that they were available in the UK officially). The larger diameter sub-base (also with that P-C opening) might come in handy as the standard base is very small and pretty tippy.
Standard guide bushes are 6mm, 1/4in (6.35mm) and 8mm. These are still widely available
TBH I have the manual on the DW670 and it won't tell you how to use the tool - the tool is a laminate trimmer and as a specialised trade tool there is a sort of implied understanding that if you are buying one of these you'll understand laminating and therefore will know how to use the tool. But as to laminating, there is naff all in print on the subject (although elsewhere on this web site I think you'll find a description of the basics that I wrote a while back - I think). The main thing to note for anyone looking at buying a single speed laminate trimmer is that they run very, very fast - the DW670 is rated at 30,000 rpm - so the diameter of cutters you can use is limited to about 12mm and no more (ideally less) unless you are keen on finding out what happens when a cutter bends or disintegrates. At those sort of speeds it pays not to fit cheap, shoddy Chinese cutters, too. The tool does have to be used at speed on shallow cuts - it cannot handle deep cuts - with care taken to ensure that the cutter isn't overloaded, which can cause it to fail
When we have some daylight tomorrow I'll take a few pics of my DW670s and the extra bases ansd post them. Sorry it's a bit long-winded, but I've been useing DW670s for more than 10 years and I bought a DW622 as my main site router about 6 or 7 years back - so I'm very familiar with both of them. If you'd like scans of the manuals, PM me and I'll see what I can do. I have the DW670 manual here, right now - not sure where the DW622 manual is (fundamentally the same except for the ratings and the collets - those are common to the DW616/DW618/DW621 USA/DW622/DW626 only)
Although I do have one of those T-shirts which reads "He who dies with the most tools wins" 
(and according to my missus I am trying... very trying 
) and another one which reads "Carpenter: noun. - A person who makes precise guesswork based on unreliable data provided by those of questionable knowledge. See also: wizard, magician" 