Do I really need to use primer when painting an unfinished concrete floor?
Probably yes.  And probably an oil based primer unless the can says otherwise.  On top of that a concrete primer if they exist.  If you don't prime then you run the risk of the paint peeling, especially since garage floors are so smooth.  Plus you might run through way more paint than you think and need to get more since concrete is so porous, it will soak it up like  a sponge. You don't have to of course, but how much is the paint vs primer? If paint is more get the primer as well.  And get an extension pole for the roller and everything will go fast.