You get a standalone version of the game. Under what circumstance would getting a steam Key through itch be preferable to getting the game through Steam directly?
Many people prefer to buy via itch so they can be sure the developer is getting a the percentage they chose, rather than one imposed on them by a platform-holder. This is one of the major attractions of itch for a lot of people, including myself.
I just bought it here because I've been excited about it forever and because sale. However, it would be lovely to have a Steam key because I'm slightly more confident that Steam will keep track of everything I own for me.