Why should I buy a car in Cyberpunk 2077?

There is no benefit other than having an additional choice to ride around in Night City.

There is a car racing quest with Claire from Afterlife, but you can always choose to pick her ride instead, and in one of the races you are even forced to.

Many of the cars you can buy are better (faster, more durable, better handling) than the ones you get for free.

Every vehicle in Night City is a unique mix in its characteristics, and many are substantially more or less pleasant to drive than others. As for not being able to customize your ride... you’re looking at this the wrong way. Buying a preem new set of wheels is how you customize your ride. No, there’s no paint shop. But with literally dozens of rides to buy or find, you’re sure to find one that suits your style.