Why do super heroes need a secret identity?

I was thinking about this last night and I really just dont get it. Like I mean I understand why someone like Stark or Wayne would have to hide who they really are, but why the hell did Superman even have to make up Clark Kent? He could have just as easily moved to metropolis and just been superman. Its not like he needs to hold a residency in metropolis or something so he needs a back-story to cover. He has the fortress of solitude and for him its only a second away.

The whole idea of the secret identity is to protect your loved ones from the bad guys but if the super hero is in a new town/planet I just cant understand the need for this. By superman making up clark kent he’s just endangering everyone who knows clark if the story were to ever slip.

Anyone got some good explanations?