When it comes down to it, there are those cars that do need to be maintained and protected from the elements and those that... Well... Don't.
Cars That Need Protecting With Carports
The cars that really need protecting are the ones that you've spent years maintaining, the ones that you've spent awhile restoring, or just the ones that you really prize and cherish and want to keep in mint condition for as long as possible. Even if that old Mustang you're finishing up still has the beat up, ugly old paint job on it, a carport isn't just for the paint job. This is where a carport can help.
A carport also helps to keep sand and grit out of the car, it helps to keep the sun from having your car overheating when it's not even on, and it keeps water out of the engine. And, not that it's a real deal breaker, but it means you don't have to put one of those stupid cardboard cutout things in your windshield to keep from burning your butt off the next time you take your wheels out for a spin.
Cars That Don't...
Let's be honest, the only reason to cover up your rusty old 1985 Chevy Celebrity "Get Around Town" car in a carport is so that your neighbors don't know how junky the other half of your car collection happens to be. Other than that, just about any car you really take pride in could do well to be given a little bit of protection from the elements to keep it looking and running like new for as long as you feel like maintaining it.