Why do supernatural creatures pretend there mortal?

Answer 1

If a supernatural creature were to pretend they they were mortal, most likely the reason they'd choose to do it is to blend in, either to get close enough to observe or attack a mortal, or just to avoid detection so as to be left alone.

For example, a vampiric creature might pretend to be human so they could sneak up and drink someone's blood, but a trickster-like being such as a kitsune might just be trying to survive in a human-dominated world.

For more on the latter depiction of supernatural creatures trying to blend in, I highly recommend you watch the film Pom Poko, directed by Hayao Miyazaki (of Studio Ghibli fame). It's a perfect and beautiful example.