There are a number of reasons why poly-coat bullets are a good option.
They do not require a lubricant applied to the bullet, like a traditional lead/hardcast bullet. The lubricant is a big part of what produces smoke when you shoot hardcast bullets, therefore they smoke MUCH less.
PC bullets create a jacket over the lead bullet inside and can be fired in handguns that traditionally recommend against lead bullet use (like Glocks).
Typically you can get better velocities with a load, using a Poly-Coated projectile, versus a similar hardcast one, or one that is jacketed. The "slippery" polymer coating allows you to achieve faster speeds, with less pressure, than metal jacketed, or traditional hard cast bullets.
And yes, to answer your question it "can" prevent leading, but leading generally is more of an issue of a projectile that is of the wrong composition (the alloy hardness is wrong) or does not properly fit the bore size. Or the gun/barrel itself has issues.
