Any reasons other than collecting $50 per year per vehicle (which is essentially a tax) would be appreciated.
Yes, part of the purpose for vehicle registration is for taxation.
Another thing that vehicle registration does is it establishes clear ownership of the vehicle (because you don't always keep it on your person.) It's kinda like the deed to your house.
It can come in handy in-case someone steals your car, or if you somehow lose your car otherwise. It becomes easier to track.
It also Identifies you, in-case you break some sort of traffic law, your license plate can be noted, and the government will be able to find whoever owns a particular car.
The reason you have to renew, is because it's easier to keep track of a vehicle if its registration keeps getting renewed. If the owner doesn't keep renewing it, It's very difficult to know whether the vehicle still exists, or if the owner (improperly) sold it without transferring ownership etc...
This question was asked 6 years ago but for whatever reason has come back up in the Top Questions list so I will answer it as no one else has properly. The asker of the question wanted
"Any reasons other than collecting $50 per year per vehicle (which is essentially a tax)",
but no other reason exists. The registration renewal with accompanying fee is used to raise funds for transportation maintenance such as road repair and construction. You can read a little more about how it works in Texas here: How it works: Where do fees from vehicle registration and inspection go?