Because some people don't throw away old hardware! A GeForce3 doesn't have a home on a Celeron system with a 440BX motherboard
WinXP is great for taking advantage of my dual Celerons while providing slightly better compatibility than Win2K. Works spiffy with 512MB RAM, but still a big performance hit.
Even then, I'm still going to have to buy an nVidia product for it just because 3dfx & ATI don't support multi-processor 3D apps & I need to squeeze a little more out of Quake3 (/r_smp crashes on all consumer non-nvidia cards

). That's what the GeForce2MX was made for (Despite it's nomination for "Most useless hardware of 2001" in these forums).