The GMAT in particular also gives people a second chance. Many people who don't perform at a high level in college develop in the years after college and are ready to perform at a high level in business school. So, scoring high on the GMAT is a way for them to demonstrate to adcoms that, even though their GPAs aren't high, they're ready to handle business school.
Meanwhile, the availability of affordable yet effective GMAT self-study resources makes scoring at any level on the GMAT possible for basically anyone, regardless of financial means.