It depends on the rigidity of the machine. An extremely rigid machine could function well being in geometric alignment but not level. A machine not sufficiently rigid could be forced into geometric alignment and secured so it stayed that way.
Since leveling and securing a machine properly probably does not require much effort, why not level it?
A level machine makes set-ups and measurements easier. May be not so much on a lathe, but lathes are the easiest machines to misalign. (twist etc.)