A play for the Bethany Moda crowd? That's really where the money is, and the music passion, but those kids will never buy physical media of any kind. Their original fans from 1985 don't care about music anymore, so U2 needs to reinvent and market to kids like Aerosmith did.
It also puts them in the news, advertising their new release to existing fans in a novel way, by making news. I'm sure they have made a record since Joshua Tree, but I haven't heard about any until now.