He's buying it for the same reason all other billionaires buy media outlets, to be able to push their narrative onto the public consciousness. Social media isn't what users post, it's which user posts the algorithm decides to show. For someone with a conglomerate with many interests and huge ambition, owning popular media can be very profitable even if the media outlet itself isn't.
That said, I think he is buying Twitter for the lulz / weird libertarian reasons, rather than making a real business out of it, and I have serious concerns it will injure what is currently a public utility absolutely beyond repair.