All the empty towers?? do yo mean the size of them?
Some towers are made for more upgradable capabilities than others.
Also, do you mean the empty slots in the towers? Older towers have slots for Hard disk drives and even older ones for soft disk drives. Also for more than one slot could be there for 2 hard disk drives.
Also the manufacturer makes different sizes... and if the same manufacturer made two different sizes, there is usually a reason.(look it up for specific capabilities on altivista)
And if youre wondering what the difference of various towers you could find at a parts store, there are a lot of differences, but not many. (someone else help, cause im a bit beyond myself in this area) But i think that different towers would have no real difference in them. The parts you install into them would be. Like the motherboard, disk drives, hard drives, etc... Also the style would be different.
I can't give you more info unless you clarify your question.