I remember reading them back in the early 90s (Robin Hood: Prince of Thieves, Far and Away, Back to the Future). They don’t seem to be a thing anymore (or am I just not noticing them).
As an aside I remember liking them but I was a preteen at the time so maybe they were terrible and I just liked the movies.
Yes, it’s common.
They can be released at the same time to cash in but the script always comes first.
The Phantom menace received such a treatment. They take the script and have an author rewrite it as a novel. In the case of the phantom menace it was the fantasy author Terry Brooks.