I've got computers from 2000 that are perfectly capable of running the programs I need, and doing the operations I need, but none of them (or even the fastest G4s from 2003) are capable of doing those things as fast as I need, and/or would like them to be.
The computer may not be worthless, but anything more than a few years old is at the end of its life for people who use their computers professionally, or for any kind of digital media, definitely appreciate the speed of any newer computer.