A Kindred demonstrates some of his or her powers to mortals or lets them partake in (usually false) mythic knowledge that makes the mortals something like "chosen ones" in their own eyes. The Kindred acts as a god, guru or the like in this, but always functions as a spiritual mentor and gives his blood to the chosen ones. The reasons to do so are as simple as the idea itself.
Blood cults are a convenient method by which a Kindred can attain vitae and gather loyal followers, not to mention the material wealth and possibilities to influence mortal society through servile and trusting mortals.
The Camarilla has made it clear that the patronage of a blood cult equals an effort to violate the Masquerade, and it is forbidden to interact with mortals in such a manner.