As with everything else the Giovanni do, it is a combination of ritual and power game - the Proxy Kiss "makes" a Giovanni: they are now aware that they are part of a family of vampires, and the rewards that await good service. At the same time, it more thoroughly binds the ghoul to the family through the enforced loyalty of the blood bond. Proxy Kisses are given as rewards for good service, and while the Giovanni don't generally let their mortal members die of old age, they might delay the Kiss for a good 3 decades if they feel would be letting their own good vitae go to waste.
Also, given the Giovanni reputation for perversity, it should be noted that the method for delivering the Proxy Kiss is an issue of paramount importance to most Giovanni, who usually make something of a game or rite out if it. Examples include an innocuous gift of wine, a parody of a Catholic mass, or simulated act of fellatio. The actual act may take some time, as the Giovanni have shown a propensity for ghouling their people first, then letting them struggle with the changes (and blood addiction) before revealing any answers.