Why do we have jellyfish?

a jellyfish is just a sea creature that dances with each other every morning and the male is the one who gets pregnant not the female! i am not really sure what is the point of it I'm trying to find it out. i never really was that interested in then but i don't know why i am not i wanted to know how big can a jellyfish get and why does it shock people?,i now realy intrested in them i dont know why but i will find out everything about them! :D