Servers are designed to optimise cooling and power at the expense of noise. They are extremely noisy, therefore, and should not be used as desktop replacements.
Not all servers are noisy. But yes, many are. Especially the rackmount variants.
However due to the fact they are meant to be managed remoteley, the built-in graphics tend to be lousy and not appropriate for Photoshop type use where you need a beefy GPU. Therefore although you think you might have found a bargain, you'll soon find yourself spending money on graphics cards.
(Although I guess the same could be said for cheap desktops too !)