You know this is really not that rare. Lolipopgi from Yogscast used to do the same thing. Full blown fuck machine, spread eagle, and taking it balls deep, but wore a bra for the longest time.
I asked her in DMs why and her answer was she just really liked to masturbate and she was doing it 100% for herself first, and was simply inviting us to watch along if we wanted. It wasn't a show for our benefit, it was for hers, and if she wanted to look at herself wearing a cute bra and not deal with a piece of clothing she had to take awkwardly take off, it was one less thing to worry about.