A couple of reasons for forward cant -
- For concealment, especially with longer gripped semi autos, the bottom of the grip will print when beneath a shirt. This can be amplified by bending or sitting depending on the ride height of the holster/gun. When you bring the gun to a forward cant it will reduce the amount of printing. To much forward cant makes for a un-natural draw, leaves the muzzle poking out behind. this is how I originally got into making holsters. I thought I wanted a severe cant for my P220. So, I made one and it sucked for me. Revolvers are not as bad, depending on the grip.
- The other reason would be what feels right on the draw from the position the holster is carried at on the waist i.e. 3:00 on the, 4:00 just back, 5:00 further back etc.
Your Serpa is not much for concealment unless it is winter and it is under a coat. However, you have the options for adjusting the cant for any of the above reasons. Duty holsters that are not adjustable are typically a straight drop or close to it in my experience. YMMV.