I like the basic idea of the Octessence are just bored and they are playing a round of complicated game of dick measuring(like literally they just want to know who is the most powerful) with the Earth's population as their gambling chips.
Beings like Chthon and Set might still have some sentimental side for Earth, it's still their home they had to fled from because of Gaea. Shuma-Gorath and Dormammu just want to conquer more territory. The Octessence on the other hand just see a fun game board to play with. Granted it's kinda similar to the Grandmaster, but the latter focus more on already outstanding individuals as his chess pieces.