Tint helps reduce the short term heat buildup and also the long term UV breakdown of the interior. I also use those windshield shields that coil into a smaller circle and spring to a larger circle/oval, one on each side.

 

It also helps keep thievin' crackhead street trash from casually eyeballing the contents of your cab. Not that that's any excuse to leave valuable things in plain sight if you're parking somewhere that those types of people like to inhabit.

 

If I parked in a place like that, I'd be more worried about my truck then whatever's inside of it.