To be honest, I've never seen my fuel pressure go above 3 psi from the mechanical fuel pump.

However, the purpose of having the pressure regulator is to reduce the pressure to around 2 psi for off-roading. With more bumps in the road and higher fuel pressure, the needle is more likely to bounce around and let in too much fuel. By lowering the fuel pressure, there is a lesser likelihood of excess fuel entering the float bowl.

For cruising on smooth roads you ideally want 3 psi to help the needle stay where it wants to be.

Apples to apples. The regulator is really more important for when you're using an electric fuel pump, since they are usually much higher pressure. Someday I hope to install an electric pump in parallel with the mechanical pump, to serve as a primer and emergency backup in case of mechanical pump failure.