I read one time that bullet lube was not a lube at all, but rather it is a sealing liquid, that helps to prevent hot gas blow by when the burning powder pushes on the bullet.
Bullet lube is also an anti flux.
What we are doing when we shoot a cast bullet, is we are pushing a piece of hot solder down a hot steel gun barrel.
Why then don't we get solder in the barrel ... well, we do ! It is called leading.
The anti flux property of bullet lube acts to contaminate the solder, barrel steel interface, and prevents leading.