instagram subscriptions are no different than the vip communities that celebs would offer on their website a few years ago imo. you pay a fee to have access to "premium" content, the difference is that it's easier to manage since it's on instagram.
I'm sure David doesn't need that income, he probably makes decent money from music, tv appearances, performing gigs, etc, but he's not exactly an A-lister who makes millions every year and the entertainment industry is difficult to navigate, it's a good idea to try to make more now because you never know what your career will look like in 10, 20, 30 years.