In Australia, if you're married as an international student, you can bring your spouse, and he or she doesn't have to study and have the right to work full time. That makes it much more easier for both of them financially, individually to make a foothold in the country. It's one part time income and one full time income to support two people and one degree as opposed to, if they were by themselves, one part time income to support themselves studying a full time degree.

Then later as they achieve temporary and then permanent residency the marriage can be used to apply for visa for the spouse.

I have met a few international student couples who marry at 20 or 21, and one of them drops out, for this advantage.

And since they're living pay check to pay check anyway, it's not risky for either of them.