While both are Archive Programs, WinRAR does a bunch of things that 7Zip does not and some things 7Zip does better, as 7Zip does a couple of things that WinRAR doesn't and a few things that WinRAR doesn't do as well as 7Zip.

Also depending on what you are archiving WinRAR does it better for various things, where 7Zip tends to work better at Raw Data and certain Data Structures.