Yep. A lot of the actions used for the BPCR game don't have much if any cam force when closing the action on a loaded round. You don't have much time for a relay 5 to 7 minutes so if a chamber gets a bit of fouling from all that black powder just go ahead and cam her on home. Can also be used to seat the bullets long and cam to engrave the bullet into the rifling. Just make sure that you do not cam on the primer.

I have one from Arizona Sharpshooters. It works well and is compact enough to fit easily in my kit.

That would depend on your chamber and on how your bullets are seated in the case.

The camming tool won't change your seating depth. It's function is simply to assure that the cartridge is fully in the chamber. It doesn't work like a breach seater.

Generally cartridge seaters are used when more black powder is added to the case; such that the bullet extends out past the point where the bullet's ogive starts engaging in the rifling, past the lead. 

The extent that it engages is dependent on the chamber lead, the amount of powder in the case, and the length of the bullet being used.

Generally cartridge seaters are being used to force the cartridge into the chamber when more black powder and longer bullets are being used; when that specific cartridge was not designed to be used with that heavy of a bullet or that much powder in the case.

However cartridge seaters are also used with less powder and shorter bullets; but with the mouth flared slightly wider than the rifle's chamber dimensions. This allows you to force the cartridge into the chamber and squeeze the mouth down to perfectly center the case in the chamber prior to firing.

Some of the tricks to either increase your BPCR velocities or better align your BPCR cartridge in the rifle to improve accuracy. This also enables you to use longer heavier BPCR bullets that are less susceptable to the wind. It also ensures the bullet cartridge is perfectly centered in the chamber.

This technique is safe for use with black powder; but using cases completely filled with smokeless powder and longer heavier bullets could cause significant problems. However if standard bullets and the correct amount of smokeless powder is used; then the flaring of the case mouth and using a cartridge seater could improve accuracy even if you are not using black powder (by better centering the bullet in the rifle lead).