Yeah, I'm kind of handwaving through a bunch of mechanisms because it's reddit and I don't care, but the general jist is multi-pronged.

Increased participation in higher education by removing perceived and actual barrier of debt burden from higher education. Increased disposable income freed up from debt payments, translating directly into increased tax base via consumption, with the spill-on effects there. Increased perceived and actual freedom of movement for pursuit of better job opportunities, etc.

I'm sure someone who's sat economics more recently than I have and have more time could make a better deconstruction than that, but that's the basic run of it.