People about Cosplay...
It’s a way of expressing your love for a particular anime/manga/video game series. You enjoy the world the author created so much that you feel like dressing up as one of the characters. It’s a tribute of sorts.
It’s fun to take a break from your real life and escape into an anime/manga/video game world, even if just for a day at an anime convention. You get to be someone else for a bit.
Great cosplayers draw crowds of convention-goers, get their picture taken by lots of photographers and receive lots of compliments. It’s flattering.
When you walk through a convention and see other cosplayers, there’s a sense of belonging to a community of people with shared interests. People make eye contact, smile when they recognize your character, ask to take pictures and are generally pretty friendly.
Some people really enjoy the process of planning and creating the costume. Making armor, sewing, and building weapons can be a fun and rewarding hobby.
What is the appeal of acting, and even role playing for that matter? Cosplay is firstly an opportunity to completely put yourself into the character’s shoes, act out a role. Additionally it is a way to showcase your craftsmanship. Just because you view something as juvenile doesn’t make it unappealing. You should be criticizing community theatre if you have that view.
Personally I feel that cosplay gives an opportunity to also put aside your own shortcomings and be someone else for the day. I feel exceedingly confident and friendly in a costume and when I see someone else in a costume, I can “know” who they are and be recognized myself. There’s a level of comradery and confidence in it. Hope that helps.
The only place it should really matter is in competition. Don’t enter a workmanship based contest with something that you bought or commissioned, and don’t take credit for the work of others.
That said, some contests have a category for commissioned or purchased costumes, you would be fine if you entered there.