Who says it is the most useless degree?
A psychology degree is necessary to become a psychologist, so if you want to do that, it is one of the most useful things you can have.
Lots of the skills learned through doing a psychology degree are relevant to a other graduate careers. Research skills, understanding people's behaviour, etc are relevant to careers in business, marketting, social research, education, counselling.
Yes I'm sure lots of people do choose psychology because they don't really know what they want to do with their life (but then lots of english, maths, physics, geog students dont know what they want to do with their life either). Maybe the people who study psychology just find it interesting.