As we all know servers essentially comes in two shapes, a pedestal and a rackmount as shown below.

A pedestal is just like a desktop casing, an upright box in which the motherboard sits vertically, only bigger. The towers need to be taller and deeper to fit the larger board form-factors required on certain server-boards. Workstations are always towers.

Rackmount servers are thin boxes that are stacked horizontally in a vertical frame, or rack, such that space can be efficiently organized when running many servers in a data-center.

Your choice of pedestal or rackmount casing is dependent simply on how many server PCs you need. If you have only a single file server to build, you'd go for a pedestal. If, on the other hand, you want to install a DNS server and 10 web servers, it makes sense to buy a rack and then mount the 11 boxes onto the rack, thus saving on space. It is not uncommon amongst ISPs or large websites such as Google or Amazon, for there to be thousands of racked servers housed in super-air-conditioned data-centers.