Non celebs do come out to family and friends though. heterosexual people don't come out because it's always been seen as the norm. Gay people come out because for some reason it's still seen as shocking to friends and family to just find out so it lets that person live openly without having to hide a huge part of their lives from those close to them.
And it's good celebs coming out because it normalises it for other gay people to come out and makes others less opposed to it.
Gay people still get assaulted by straight people for being gay fairly often, so the more it can be normalised and stop things like this happening the better.