The type of computers you use in the office naturally depend on the kind of service your company is offering to people. If you are working in some accounting firm, I guess having a regular desktop and a couple of laptops would already be OK. For computation-intensive work or work that requires a database, probably having a mainframe aboard is the most conceivable, partnered with a couple of desktops to serve as workstations for the employees.
When your office is filled with desktop computers, chances are a lot of tabletop space is used for computer equipment: CPUs, printers, scanners, etc. While the space is not really 'wasted' as they still serve some purpose, it could have been put to better use if the other computer stuff weren't there.
So how do you go around this? I can see two ways. First is to use laptop computers instead of desktop ones. Laptops can be easily carried around, apart from the fact that they save a lot of space. It should be noted, though, that laptops generally have lower computing power than their desktop counterparts. They can be also pricey--probably too much for you or the company you are working for to afford.
If using laptops isn't possible, then you can still save tabletop space by--what?--using space under the tables! The space under your table are hardly used, so why not put everything that can be put there in there? For these purposes, using this CPU Holder may help a lot. Putting up a box under your table where you can throw your trash will conserve a lot of tabletop space as well.