From what many people seem to like about shoulder holsters is the stability of carrying a larger size revolver or duty gun.  Something like a Beretta 92 or 1911, Sig 228 ect are all a little large for IWB and OWB limits the defensive movement inside a vehicle.

Galco holsters come in a very diverse variety.   You you use them for concealed carry purposes or for open carry or duty.   One of my favorite ways to carry a 1911 is a Galco shoulder holster.   I like the idea of being able to sit down in a car and still be able to grab the firearm without fiddling with a seatbelt.  The only other car friendly variation of carrying a firearm in a car and being able to reach it quickly is with an ankle holster.  Galco has several shoulder holster and ankle holsters on the market.