I have... opinions on accessories, but it basically comes down to immersion > practicality. If it would make sense on a real-steel platform, it tends to go on the airsoft platform, whether it's actually useful or not. I'm an immersion nerd who pretty much solely uses GBBRs and gas bolt actions with tiny mags.
There are attachments that simply do not get used very often at all; my non-bolt action rifles all tend to have weapon lights and laser modules if they are able to, though the lasers can only be used at very specific events in a limited capacity, and lights only see very occasional use in night games or CQB (which I very rarely do); often I don't even have batteries in them, but they stay on for the immersion purposes. My handguns all have lights on as well, again often with no batteries in them, once more for immersion, a little bit for practicality if I'm going to a site or event where a light may help, but also because my holsters require them now, so they also never come off.
Slings are non-negotiable. If I'm carrying a rifle, it has a sling on it. I don't have independent slings for every rifle (yet) because good rifle slings are expensive, but I never leave home without one. I'm also now starting to learn and properly use slings in the same way people do in the real-steel world and it's great. Not as much of a fan of pistol lanyards though because modern holsters are great, and it's another thing to snag or get stuck, or potentially slow down my draw which I do not want for a handgun which is meant to be an emergency weapon (or a sub-30m MED shooter where draw and aim time is also very important)
Optics are all chosen with a purpose. All my bolt actions and DMRs have mid-power variable optics as with a 30m MED, 3 or 4x minimum magnification is fine, and they're not designed for quick shooting. These are often not light, having 50mm objective lenses and 30mm tubes (though my Tanaka AICS has a 56mm objective lens with 34mm tube and is really heavy), as I chose them to be comfortable for long distance shooting from a position, with clear, high quality glass, great eye relief, and illuminated FFP reticles. People always ask "do you really need 24x zoom?" but when you're shooting at the extreme edges of airsoft ranges at UK power limits, it's incredibly elpful to be able to see exactly where the BB is dropping, or how much you're missing by so you can adjust.
My scout / recce rifles have low-power variable optics (apart from the Tanaka G33/40, as that's got a 1.5x ZF41 scope because period weapon!) as they fit that middle ground between fighting rifle and SPR, so a bit of zoom is really nice (and I sometimes run them as DMRs in airsoft terms), then my one fighting rifle has a red dot because they're fantastic for quick target acquisition and as long as you're in range and you have taken the time to zero the optic, if you line up the red dot on a target and pull the trigger a few times, that target will likely put their hand up and shout hit soon after. I did have a red dot + magnifier combo, but repro magnifiers are absolutely awful for eye relief and I just didn't get on with them. My fighting rifle setup is a 10.5" AR anyway, so red dot is fine even from an immersion stand point.
I also think that 45 degree offset sights are basically pointless in airsoft because of how hop up works, but I also don't feel the need to have one on any of my rifles. If I want a red dot alongside a scope, I tend to just mount them high, on top of the scope rings. Plus, with the aforementioned light / laser attachments, a 45 degree offset red dot is probably just going to be looking at the back of the MAWL-C1+, or the Surefire m600 mounted on that side of the rifle, or the edge of the PEQ / DBAL. It's just not great.
Then the under-barrel attachments...
Bipods. A controversial one as most people say they're wasted weight, but this fits into a mix of useful and immersive for me. I don't have any bipods on my fighting rifle or scout/recce rifles. For a fighting rifle, a bipod is absolutely pointless, though I've definitely seen airsofters with a great big bipod on their 416 replica and always wondered "why?" And for my scout/recce rifles they also don't have bipods because the whole point of those rifles is to be a bit lighter (forget that one of them is an m14 in a real wood stock with a full steel build...) and you're not sniping with these rifles. They're designed to be long range capable, sure, but they aren't SPRs. I find if I do set up in a more snipers hide style position, I just use my backpack as a rest (which again is what a lot of people do in the real-steel shooting world). Then all my bolt action sniper rifles (not the Kar98k because no...) and my dedicated SPR build have bipods. They're more deliberate shooters, and honestly the bipod is useful more often than not. When you're set up in a position for an hour or so, it's so good being able to just deploy a bipod and not hold your rifle up for the full hour or so. It's not too much more weight and is definitely worth it for those guns in my opinion.
And grenade launchers are entirely not my thing. I had one, but I never used it. I liked the idea of using one as a spotter, since the US Army scout sniper handbook that I found (older one from the 90s, widely available because it's not current military practice) talked about using an m203 to break contact, but I didn't want to shell out for tag style shells (tag products being banned at a lot of sites anyway), and shower shells are just not that useful. I tend to just use one of my scout/recce rifles if I'm playing as a spotter, as my paired sniper will be taking those long range shots and I'll be spotting targets, relaying distance and wind speed (yes, I have a proper rangefinder and a cheapo wind speed reader because immersion, but also because it makes us better shooters to try and learn some semblance of consistency), and providing security for any threats within that 30m MED, so I want something lightweight but still capable at both medium and short ranges. Instead of grenade launchers, I way prefer keeping things light and using some sort of front grip, whether angled or vertical, to just make it a bit more comfortable and ergonomic shooting the rifle.
