For some weapons, definitely yes.
Have a look at the equipment in SG Manual, where “Primary Fire” means shooting with left mouse button and “Secondary Fire” with the right one.

Pistols: Remington 1858 Model, Smith & Wesson Schofield, Colt Peacemaker:
With secondary fire you shoot faster (though less accurate than with primary fire) leaving your opponent no time to counteract.

Shotguns: Remington 12 Gauge, Sawed-Off Shotgun:
Secondary fire = double shot (both barrels).

Bowie knife:
If you have more than one knife you can switch between slashing and throwing mode with the secondary fire button.

Dynamite Stick, Molotov Cocktail:
Secondary fire is used to light them.

Gatling Gun
Secondary fire disassembles the Gatling gun so you can carry it with you.

Posted in: Smokin' Guns — Basics