You ever hear of a SF writer named Alan Nourse?
{That's Doctor Nourse, pronounced "Nurse". He did his residency, apparently, at ah hospital where they had another resident named "Dockter". I understand the PA announcements were amusing...}
Anyway, Alan Nourse wrote a book named Bladerunner.
The movie had nothing to do with his book, but they paid him a chunk for rights to the title, anyway.
As to why they bought those rights, I can only assume that it was to avoid the possibility of someone making a movie based on the novel, which then would have had a very similar title. That makes sense, since someone had already made one unsuccessful attempt to do so. With such an odd and unique sounding title, audiences surely would have been confused, and that's not good for anybody. As to
how one goes about "buying the title" of a book, I'm a little unclear on what exactly that means. It may mean that they bought the rights to the entire book, or that they bought the rights to adapt the book into a film. What I do know is, I've researched this multiple times, because I've had this argument many times, and every time I look into it, I see the same thing consistently everywhere I look, including on the U.S. copyright office's own website: it is not possible to copyright a title, and you cannot stop someone from using the same title that you have already used, except in very specific or unusual circumstances, such as if it is the title of a series which then gets into branding laws, or if it's a made-up word that you have trademarked as a brand.
And there is good reason for this. If everyone who used a title or business name that had already been used had to pay the first person smart (or lucky) enough to use that title, then there would be constant lawsuits and a bunch of really wealthy people who got money by publishing an unknown piece of garbage with a catchy title. Every person who used their name as their business name would have to pay the first person ever to use that name. It would be hell on earth for the copyright offices, and they don't wanna deal with that crap.