I came out ti to everyone in school at the end of year 8, and I'm starting to regret the decision. It's not as if it was a painful thing for me to realise I was gay, I just realised I love men and got on with my life. So why do people do it?
I think it's to do with acceptance perhaps knowing who accepts it and who doesn't but I know what you mean.
A lot of people come out so that they can gain support from other people that are out, so that they can start dating, so that they can tell their family and they don't have to bottle it all up.
Lots of reasons. You want it public so that other gay/bi people know, since most people assume straight until proved otherwise. You want to give people time to get it through their heads so they don't say something embarrassing/offensive or don't get really weird when you eventually get a date. You want well meaning people trying to introduce you to random single people of the opposite sex and/or want to encourage well meaning people to introduce you to random single people of the SAME sex. You want to find out in advance who around you is going to take it really badly, so you don't end up getting unexpected abuse when you eventually meet somebody. You want people to hear it from you so you can get rid of that damn "Did you know *** is gay now!" rumour. You want to be able to announce "OMG in a relationship" without being bombarded with "Since when are you a fag lol!" like everybody else does. You see it as part of embracing who you are. You say it to the world not because you want them to know to about it, but because you want them to know you embrace it and aren't ashamed of it. You're sick of sitting in the damn closet! And probably many, many more...
Well that covers maybe two of the things I said... The point of coming out is different for every single person who does it.