@Femme,
A chief reason for
coming out, that is announcing a sexual preference, would be that society as a whole, has been known to disapprove of what it has deemed to be
"normal ", normal supposedly being heterosexual.
By stating that one is bisexual, homosexual, asexual or transsexual or anything else which may meet with resistance, it both makes it become fully integrated and normal to people and helps a person who (because of the narrow-mindedness of many), has been to timid or scared to let anyone know, or live their life to the fullest by embracing who they are.
In time, this may no longer be an issue, it has already lessened greatly from where it was 40 or 60 years ago.