What is the point of teams in NASCAR?

It gives the car owner more than 1 bullet in his gun. One car crashes out or has a mechanical problem, it's not the end of the day for the car owner.

Each car can share setup data and compare notes, which theoretically should make each individual car quicker or better handling.

Testing is limited, so with more cars comes more data.

If they have motor or part problems, with multiple cars it becomes much easier to pinpoint how and why the problem occurred. Example being if a valve spring failed on 2 team cars in the same race, they have a pretty good idea that they better dig into why it failed. They would tear the motors down and compare the failures. On a single car team, it could have just been a random failure and they may go down an unneeded path in research and development. Multiple cars really helps validate theories, new parts, failures, etc.

To elevate each other & thereby the competition & sport in general.