If you order a wheel and tire package that will fit your car, do you need wheel spacers? what are these? what is the purpose of them and where do they go?

The would essentially push your wheels out farther than they are.

If you get wheels/tires with a larger offset than the OEM set, the front of the wheels would sit inside the fender more than they do now (depending on actual wheel width though) and wheel spacers could bring them out towards the fender more.

They're also often used to make the wheels flush with the fender since most of the time, the OEM setup has them inside the fenderwell.