the main benefit back in the day came down to better efficiency on period CPUs, since it's a lower level API than the other ones, relatively speaking. since it's probably very common to go with quite fast setups now this fact won't even be realized generally, so don't worry about needing to buy a 3dfx card.
to give some examples from my own testing, GTA 2 has absolutely zero frame drops on a p2 233 in glide on a voodoo3, and that's basically close to the minimum CPU requirement they wrote on the packaging. run the game in D3D with the same card on that same setup, or any other (faster) card really, and you'll get some pretty heavy freezes when driving around.
diablo 2 is a very similar case: on a p2 400+v3 it's quite solid in glide with only a bit of slowdown once the entire screen is filled with enemies. moving to a p3 1100 gets completely rid of that and the game pretty much always runs at its 25 FPS cap. now, swapping in a much faster card like the geforce4 ti 4200 with D3D, suddenly there's quite a bit of slowdown even in relatively light areas - and that's with a CPU that's already from a year later, basically. i've not tested D3D on slower setups here but it's not going to be pretty. i'd expect that really at least a decent athlon XP setup is needed to run that game at its cap in D3D.
in both cases i believe the games still look fine with D3D, at least on period machines, it's just that the overhead is jarringly higher. to what degree this actually comes down to the differences between the APIs or the game progamming is debatable. there's actually a couple of D3D games from that time that i consider to be fairly efficient, like forsaken, but with most games that had both D3D and glide modes, there's a pretty consistent theme.
incidentally, the voodoo1 in its early period was quite overrated though, IMO: glquake/glhexen2 didn't even achieve feature parity with the software renderer (read: overbrights), and a lot of early supported games were just quick ports with bilinear filtering pointlessly tacked on. i'm still not sure what the point of that shadow warrior 3dfx version was.
AngryByDefault wrote on 2021-06-05, 13:57:
Is a Glide version of a game very different than a D3D version? (i.e. Unreal T. on Voodoo vs Unreal T. on nVidia)
UT99 has some major visual glitches in its original opengl renderer, like parts of the redeemer first-person model being cut off. i believe D3D was mostly fine, as by that point they had done a lot of work to get their engine running properly on that, but it still always seemed to run with some pretty washed out gamma by default, in comparison to glide. and obviously the point about overhead still stands here, and even more so as that's a quite CPU heavy game for the time on any renderer. for playing that game now you can just go with the utglr renderer and get the added benefit of s3tc textures.
now unreal didn't even ship with D3D originally, and when they patched it in, it notoriously was quite compromised graphically at first. it's really just a historical curiosity now, but if you look up some old nvidia TNT reviews and such, you can still see those missing chandeliers.