Why do they say if you get good grades in school you can get a really good job? dont you need exp as well?

I have been talking with family about school and how grades are important. They say that if you do good in school you can get really good jobs? How is that? Do employers look at your grades when you apply for a job? I mean my grades arnt that good but I am pulling it up and probly transfering to another school to have a fresh start and have had decent jobs at companys so far so my resume is decent. How can getting my grades good land me a good paying job? My brother is making alot of money now but his grades are horrible. Do grades really matter too employers?