You can strop on leather without compounds. However from what I have gathered. The leather needs to be properly cased. Which is where it is very lightly dampened, then rolled with a heavy rolling pin, or something else round. This helps bring natural silicates to the surface, which is the very fine abrasive naturally occurring in leather. This is abbreviated, Stitchawl is the knowledgeable one on strops.
Compounds can be applied to help polish the edge, and refine it more. Different compounds cut differently. I personally like the edge diamond abrasives leave. A very popular one is Chromium Oxide, which is a very economical abrasive. And lower grit stropping abrasives can help bring your edge back to sharpness instead of going to the stones, when it is only lightly dulled.