The career world is like an office building. If you quit your education at high school, chances are you’ll only get to the first or second floor and not much farther. If you want to get somewhere good, you will very likely (but not always) need some sort of advanced degree.
Those good grades allow you to get the education to take the elevator higher in the career world from the very beginning of their career.
For many floors, GPA doesn’t matter as long as you have the degree in question required to establish your competency to do the job. Even if you have a stellar academic record, you still need to ace the interview and impress people with your actual skills.
Some people manage to get rich, own their own companies and/or run giant corporations without any or not much formal education. There are exceptional people and some people have very little control over what academic opportunities they can pursue due to their life circumstances. But they still find a way to apply themselves and do great things in the world.
For the majority, however, doing well in school and getting advanced degrees or professional degrees is the ticket to a safe, high future income. This is not guaranteed because professions and occupations go through variable cycles as the economy rolls through the business cycle (which itself is changing in nature as time marches on).
Also, good grades in any program will allow you to back to school easily later in life if you decide to pursue more education or if you decide to switch career fields. So it just makes sense to apply yourself to your studies and extra-curricular activities which build your resume. You should take full advantage of any and each shot your are given to prove yourself competent and intelligent.
Best of luck to you...
P.S. As stated by another poster a lot of American companies ask for a minimum GPA level and proof you have achieved this.