IMNSHO
In addition to easing the projectile's travel down the barrel, lubes do three things for you in a Cap and Ball revolver.
1. Keep the pistol shooting during your day at the range by maintaining free operation of the moving parts.
2. Act to keep the fouling soft so it can be easily removed during the cleaning process.
3. In some cases (the absense of a wad under the ball) seal the chamber to prevent hot gas from the "in battery" chamber from migrating to adjacent chambers and detonating a chamber that is NOT in battery
You will get probably ten different recipes for bore lube as additional posts on this thread. Nearly everyone will recommend that you make your own rather than spend a lot of money for a commercial product which (Also IMNSHO) is inferior to what you can make. One commercially available bore lube is Thompson's Bore Butter. Some folks like it but I think it stinks both figurativley and literally. Most of the cost of manufacturing Bore Butter goes into the useless disgusting perfume they put in it. I have had a half used tube of Bore Butter in a drawer for about four years. I think I'll try it in my egg nog on Year Years eve.
I like approximately half and half Crisco and wax rings from under toilets. During the summer I use more wax ring than Crisco. Beeswax will figure heavily into some of the recipes and I am about ready to admit that it has its advantages in spite of the fact that it is expensive.
In addition to this home brew lube, I make sure to douche down the internals with Rem Oil (Walmart) after I clean the revolver to fend off corrosion and lubricate the internal mechanisms of the pistol.