"Upgrade? What's bad with those 15' CRTs?"
A bit exaggerated, but not so far from reality in my job.
But when they really upgrade some PCs, they buy a few at a time, to upgrade some of the IT departament first, then customer support. The older one is often given to a new employee or to another one, replacing an even older computer with it (kills two birds with a stone), and that one ends up in a corner, collecting dust.
They might serve as a backup PC if something breaks, keep for spare parts or a few times they were donated to a nearby school.
Not to mention my personal graveyard of older hardware
Some of it may still work.