According to the Mark ShuttleWorth inaugural blog post about Unity, at the beginning Unity was designed to create an interface that would be usable on variety of machines (standard computers, netbooks, light versions)
A user could start his Ubuntu's experience on a tablet/netbook, then switch to his desktop and keep the same interface, Unity of the interfaces ;) .
Another reason may be the way that the applications are docked together. The runnable (launchers) and the running applications are merged in the same icons in the panel, that can be another way of Unity as well as the open Windows' toolbars being merged into the panel.
And last reason, as said on the unity website,
Unity is designed for mouse, touchpad, and keyboard use
again, no matter the way you can control your machine, you'll find Unity in the way it is managed.