Two permanents (or a permanent and a spell) are traded between two players, and each player gains control of the others game piece
Being able to make non-legendary copies is always great for any MTG commander that you'd like to have two copies of, and in a fliers deck or something, your board will get very big very quickly
Multi-typal support is a great feature for MTG to have, and it solves a lot of long-running problems with typal decks in general
(For example, you can target the same player with Toil that you target with Trouble, or you can target different players.) EDIT -- One other thing: When a fused spell resolves, the left half's effect happens first
Look, Ive been there
These cards are also made to work well with the Secret Lair x Sonic: Friends & Foes set, like teaming up Knuckles with The Reaver Cleaver to turn it into Knuckless Gloves for an unexpected combo