From The New Yorker:
Society needs a mechanism for sorting out its more intelligent members from its less intelligent ones, just as a track team needs a mechanism (such as a stopwatch) for sorting out the faster athletes from the slower ones. Society wants to identify intelligent people early on so that it can funnel them into careers that maximize their talents. It wants to get the most out of its human resources. College is a process that is sufficiently multifaceted and fine-grained to do this.
College is, essentially, a four-year intelligence test. Students have to demonstrate intellectual ability over time and across a range of subjects. If they’re sloppy or inflexible or obnoxious—no matter how smart they might be in the I.Q. sense—those negatives will get picked up in their grades. As an added service, college also sorts people according to aptitude. It separates the math types from the poetry types. At the end of the process, graduates get a score, the G.P.A., that professional schools and employers can trust as a measure of intellectual capacity and productive potential. It’s important, therefore, that everyone is taking more or less the same test.