They increase penetration by both reducing the cross sectional area (small effect), and by allowing the blade to cut on the top as well on the bottom (huge effect). As well they allow you to drive upwards and/or turn on a stab after the initial penetration much easier than a blade with a full spine.<font face="Verdana, Arial" size="2">What are the advantages and disadvantages?</font>
They also provide another cutting surface which can be used in self-defense applications as well in regards to utility for heavy impact work where you want to save the primary edge, or the reverse if that is your preference.
As for the downsides, they lower the strength of the tip, how much so depends on how deep they are ground. They also hinder using the blade for a lot of applications such as a draw knife, and a splitting wedge and overall increase the damger of use.
They also tend to make the blades look "tactical" and like "fighters" which is a $good$ thing considering the current market.
They usually don't have secondary bevels unless you are taking about swedges so deep they are actually approaching being a dual edged blade.