I agree that it depends on your relationship with them. If you feel it would help them to understand, and that they'd be supportive, then I think it can be helpful. It sounds like you've decided to tell some of them, so I really hope that it goes well and you find it a positive thing.

I don't think it's always necessary though. The couple of people I have told have just been confused by it, and although I have good relationships with my friends and most of them know I've had "issues" in the past, I don't really feel the need to discuss my mental health with them and I think they would also be confused if I told them about it. I'm okay with that, I don't feel that I need anything from them that I don't already have, or that any of us would gain anything from me telling them about my diagnosis.