Many things. I mean, you could hack a lot of stuff into Excel but generally

SQL has foreign keys and integrity checks. You can make it so like if you delete a user it automatically cascades to delete other rows like their addresses.

You can prevent someone from entering the wrong type of data in particular columns. This one’s an integer and that one’s text.

It’s designed to work on larger scales. Excel stops at 1 million rows per spreadsheet, unless my search just gave me AI slop.

You can do queries, for selecting as well as updating and deleting. You can join tables.

It’s much easier for other applications (such as a website) to talk to a SQL database

You can do transactions.

There’s a lot. That’s just off the top of my head.