I think it would largely be for looks, especially with silencers, they don't even do anything for airsofting.
I wouldn't say suppressors dont do anything in airsoft, my old, now sold stubby-sup worked quite well on my G36, being mentioned about its effectiveness and questioned on where i sourced it etc. - it actually made it sound quite authentic.
as for accessories, most of it is 'pimp-age' and 'bling' to be fair, non of the lot i know at least really use PEQ boxes for their proposed function as they only generally hold batteries - UNLESS you buy the modded/ more expensive ones with laser and torch - but then how many actually use them for their intended use - could only think they were useful in CQB, and then only really for the torch :/
to be fair, when i go, its mostly the younger 'COD' players that bolt everything and anything to their guns to try and replicate the game - which is fair enough.
Personally, it used to be the case of bolt this and that on in attempt to make my gun look like a Grim Reaper but now its functionality. - simple RIS rails, Verticle Grip, 552 and off i pop.
This picture was back in 2008 when i first started, I sprayed my gun grey, bolted a torch, peq box, v.grip and red dot on - of the 4 i used 1 - the red dot lol!
- just my 2 cents worth