There are a number of reasons to want to tint your car's windows from UV protection to style. There are laws regarding window tint, however, and they vary pretty wildly from state to state. There are also different kinds of window tint, and different ways of applying them. You don't want to get saddled with annoying tickets, or end up with a crappy, job filled with bubbles and wrinkles, so paying attention to this stuff is certainly a good plan.
It's the kind of thing that you want to do some serious research on before comitting to a product, and this video from /DRIVE really covers the ins and outs of this particular modification.
Fun Fact: In most states you can get a prescription from an optometrist or physician for window tint that exempts you from the legal limit.