Length of credit and credit utilization, you get points* for the length that each account has been open, so when you pay off your loan the account is closed and no longer counts. Also as you get to the end of the repayment it shows as a $30k account that you owe les than $10k on, you get points for using less than half or less than a third of the credit available to you.

*You don't actually get points, that would be too easy to understand, you get factors that affect a complex equation in your favor.

Credit scores are in part based on the oldest line of available credit, which for most people are their student loans. Pay those off, your oldest line of credit becomes something more recent, and your score goes down as a result

It decreases your overall available credit