They get relatively cheap, more educated workers for at least a short time. And they're often able to keep them at a cheaper salary than hiring someone with the same education. A (proactive) promotion that doubles your salary from $35 to $70k a year generates a lot of goodwill, even if that education and position would usually start at $90k.

Also people who "go to college" that work pays for don't live on campus, so the company is only on the hook for tuition, and not room and board. And it's often not full time. It's worth $10k/year for all that.