There are a lot of very successful people that dropped out of college, such as Bill Gates (Microsoft founder), Steve Jobs (Apple founder), and Mark Zuckerberg (Facebook founder).
For these people grades did not matter one bit, but they were incredibly talented and also had good timing/luck. Getting good grades in my opinion is like a nice safety net, just in case you don't make it big on your own. Good grades will get you into a better school which can lead to a better job and life. If you are not lucky enough to start the next Facebook and don't have good grades, you could just wind up on unemployment or having to work at a low paying job with limited opportunity for advancement.