It's the same as with cars - companies want brand new because then you get a full warranty and theoretically you don't need to worry about it. So that $400 you saved would be spent in IT support for your old failing hardware.

But the thing is.....old PCs are really not that unreliable. If they survived the last 5-10 years then they are probably still chugging along just fine and for a small business there is literally nothing wrong with using them.