Is this a Sonar Artist vs. Platinum type question? Or are you talking about the extra prchannels that have to be bought on TOP of Platinum?
If you are on Artist and wondering why Prochannel and the modules included in Platinum (or Professional) it's really quite simple.
You get to use them like a mixer channel strip AND they are very useful/nice sounding plugs. They are either just there or easily loaded/accessed/configurable instead of having to insert and manage inside the FX Rack/Bin. I just select the track, hit I and I can tweak the PC modules I have active. If they were in the FX bin I gotta open the individual GUI of each pluging everytime I want to tweak something (which is kind of a PITA).
And the quality of the effects is really good. They sound good and are easy to use. The VST/insert style effects are still useful and needed but once I got used to the PC effects I hardly put anything into the FX Rack/Bin anymore unless I really need to.
Also the PC EQ has a real time frequency analyzer that's super handy.
You'd really have to play with it a bit to fully appreciate the usefulness of the PC but seriously it is a huge time saver and the plugs sound good (and they are better/different than the included VST plugs in a lot of cases).
If you mean the PAID extra plugins like the CA2A, Concrete Limiter and the various other PC addons... IDK entirely but I have purchased the CA/PC2A and the Concrete Limiter and yeah... they're really good. The other stuff I don't own so you'll have to ask others.
Cheers.