When you buy a new cast iron skillet, the first thing you do is put cooking oil in it, and place it in the oven until the metal seasons. This keeps food from sticking and gives the skillet a protective finish that lets you clean it a lot easier.
Thats basically what I use Bore-Butter for when breaking in a new powder gun. By smearing the metal with bore butter, then placeing it in the oven until the metal gets hot (not so hot as to hurt the metal) will make the weapon easier to keep clean and will result in less break in time at the range.
Most guys that shoot conicals or shoot patch and ball, use it to lube the bullet or patch. To much lube on either can result in a fouled powder charge, since bore butter is water soluable, and heat will also make it turn to a liquid. Use it sparingly if you intend to hunt in warm weather.
I shoot sabots so I don't use the stuff at all except to keep in the barrel between seasons. The dryer the barrel is, with sabots, the better accuracy you will have. If you lube a sabot, it slides to easy out of the barrel and your riflings don't function the way they should and will result in poor accuracy.
There is no down side to useing bore butter to season the barrel of your powder weapon or to use as a patch or bullet lube, just use it sparingly.
It is vegetable grade and will in no way hurt your weapon like petroleum based products will, if used with black powder.