Grades are correlated to intelligence. Smarter people get higher grades and dumber people get lower grades or drop out. Sometimes people fall through the cracks but this is generally the trend. In the end the people who get higher grades end up making more money than the people who get lower grades and drop out. Sometimes this isn't the case, but generally it is.
You'll find that the professions that make the most money--Wall Street traders, doctors, lawyers, scientists, programmers, engineers etc-- are regarded as smarter than average and generally receved higher grades in school versus the norm.