1. It subsidizes institutions of "higher education."
2. It grew rapidly after 2008 because the home mortgage market collapsed, and banks needed other customers who would be willing to borrow enormous piles of money for "assets" with inflated values.
And the institutions knew it. All through the 80s and 90s I watched the annual price fixing scams of the higher education industry as they raised tuition and room and board in lock step at rates which far exceeded any measure of inflation. I wondered why would any potential consumer put up with it? The answer, of course, was the easy availability of low cost student loans students would resort to to makeup the shortfall. Back then it was also relatively easy to walk away from the debt without impact so tons of people did it.