Nothing, I have 96GB of ram in my server, upgraded from 24GB... it sits somewhere around 5gb of usage and sometimes spikes to 6 or 7, lol. I just bought it because the price was right and I wanted it. If I get bored I could probably make a nice size ram cache for my disk i/o... but even that is limited by my network speeds anyways. I'll probably never find a good use for it, but if I do, it's there for me!
It doesn't take much ram. but back before SSDs were cheap, I made a 4gb ram disc to run my minecraft server off of. Ran incredibly well, no lag generating new chunks regardless of crazy shenanigans going on.
Ram disks are old news now but it felt real impressive at the time lol.
RAMcache. Absolutely love it. You know how you hear that NVME's don't make loading any better? Well RAMcache's absolutely fucking do!
Otherwise just having a shit ton of tabs open in the background combined with some of those simulation games that use a fuckton of RAM (Cities Skylines + Mods, Planet Coaster/Planet Zoo, MS Flight Simulator 2020 etc.) and its gone pretty quickly.