Sony suffered a data breach in 2014, and all their emails leaked. In those emails, they were discussing plans to make a Sinister Six movie with Spider-Man having a major role.
No, the Sinister Six movie eventually became Spider-Man: No Way Home. The reason they keep making the SSU movies is because they want to have their own billion-dollar grosser all to themselves without sharing the profits with Disney.
Marvel licensed the film rights to Spider Man to Sony.
Then X-Men to Fox.
Then Hulk to Universal.
And throughout all of this, the lawyers have fought over which villains or characters properly fall within each category, signing new deals or borrowing characters and rights.
The Disney-Fox merger made things simpler for X-Men versus not-X-Men characters. But the Spider-Man cross licensing for Sony-produced Spider Man movies that take place within the same universe as MCU makes it more complicated, too. So did the Netflix rights to Daredevil and Jessica Jones and a few other characters in that orbit.
Wtf is it for?
To make money, including making sure that rights don't lapse from non-use.