When artists get too big they start making all kinds of crazy demands. They want to feel like real bosses and not just studio gangsters or young pretenders!
For this reason, a major record company may decide to give them their own vanity label or stroke their ego with an imprint.
In some scenarios artists, themselves may feel that their record label is not putting enough money into their projects so they may negotiate to have a vanity label where the artists make certain financial contributions while the parent company does the same to cover expenses.
Of course, on the grounds that the parent company will not interfere too much when it comes to the creative direction the artists have.