Never flew on Delta, how does this help if you are flying partner airline in economy?
You get skyteam elite plus status. Main benefits are
Lounge access
Sky priority (check in, security/immigration, boarding, baggage handling)
Extra baggage allowance - usually one extra free bag
Preferred seating
Priority on waitlists
Depending on the airline you might get some other benefits, like upgrades to first on certain Aeromexico flights.