I don't think grades are everything, but the work you do to get them means something. It's being disciplined and a hard worker than matters. I know there are tons of well off people who didn't make good grades and that's fine. There are some people with a certain ability for success that are going to do well no matter how much education they have.

But... if part of the terms for a reward are getting good grades, then you do it or lose the reward. No one has to buy their kid a car, no one has to even let their kid drive, but parents sometimes do this for their child when the kid shows that they are deserving. And if they don't think they deserve it, it's their money and their decision.