Single player uses:

1. Taming mount: Karkinos is pretty tough and unlike most "big dinos" you can use a weapon while riding it, they can also grab two dinos at once and if you dismount then they won't immediately eat them like a Megalosaurus would.

2. Aquatic mount: Karkinos can't be grabbed by a tusoteuthis and are very strong.

3. Power mount: Big crab strong.