Games and pure nostalgia. I was born in 1990, so during the time of my first memories Windows 3.x was in it's hayday and DOS games for the 386 and 486 were rampant. I played DOS games and Win3x games (pipe dream, woot) up through when Win98SE stopped being viable for the latest games (2005 - yes I used 98SE until then on my main machine, really). Of course I still play DOS games, but now it's mostly on my Compaq all-in-ones. I am planning to buy that RAM patch so I can put 98SE on my main machine in a dualboot with Win7 x64, heh.
Anyway seeing saundby's post perhaps I should elaborate more.
I have a maxed out Pentium 3 (1ghz) Dell box that WAS a l550r iirc, but now displays l999r when it boots.
It's got 512MB RAM and ran Win98SE but at the moment It's in a corner with no OS (aw.. :/ need to get that machine back up and running..). This box is for games that are too much for the below Voodoo machine to handle, and I hope to eventually find a PCI Voodoo 3 for this one so it can do the later Voodoo stuff.
I use an overclocked mobile-Pentium that was converted to a desktop chip in China that I run at 300mhz (was rated 266mhz) with a Voodoo II (Diamond 8MB) for Win98 and 95 games that won't run on a modern system (or won't run as well). This machine is named Voodoo - I even got a 3dfx case badge for it.
I realize that AMD chips of that era (K6-III and such) would be much faster and less limiting to the V2, but I enjoy the untranslated feel of the Pentium so opted for the fastest one of those I could find, leaving things that require more to later systems.
My Presario 425 (likely to be replaced with a Prolinea [same but with built-in net, freeing an ISA slot]) is used for the majority of DOS/Win3x gaming, and likely some Win95 gaming (early stuff) too soon enough. This has a SB16 and an Etherlink III at the moment in it's two slots, and I run DOS 7.10 on it.
I don't have a 386 platform yet (I do have a board or two around, if I needed one for some reason), as typically things that were designed for a 386, 286, and even 808x run fine on a 486. However, I should be getting an interesting 50mhz 386 system soon from glitch (as well as the aforementioned Prolinea).
For 286 (though I typically use my 486, as I said) I have an original IBM 5170 AT. It's got a VGA card at the moment, but I'd eventually like to get it going with EGA and a 5154 for games that might require a true EGA card.
For 8088 (again, typically use the 486) I have an original IBM 5160 XT. It also has a VGA card currently, and I'd like to kit this one out with CGA and a 5153 for the same reasons, but for CGA games. I also have the Panasonic Sr. Partner for this - though it's screen is monochrome, so it's not ideal for games (I treat the Sr. Partner as an arcade machine - I play Frogger, Gremlins, Galaxian, etc.).
For 8086, which probably has not a single game that won't run on an 8088, I will be using the NEC APC (also from glitch soon enough). If I understand correctly this machine might not be graphics-compatible with era games, but I'll likely relegate it to text-based games for this very reason, and also for games specifically written for the APC (because those do exist!).
This list consists of one or two machines from each category, so bear in mind I do have other less frequently used machines. One can't use every machine on a constant basis, obviously, which is why I usually stick to my 486.