A strop can be used without any compound. Sharpeners of straight razors for instance often prefer using a smooth leather strop with no compound applied. The leather polishes the metal and removes any burr from the edge, leaving it crisp and sharp.
Knife and tools users on the other hand often use a compound on their strops. They find it quickly and easily gives them an excellent edge that meets their needs.
In our experience, whether or not to use a compound is a matter of personal preference. Excellent results are achieved with either method, although honing compound with its abrasive particles will show results more quickly than plain. In the interest of being thorough many of us do both, using one strop with compound applied, then following up with a few strokes on a plain strop.