You do not need one, but it is the cheapest thing that will keep your knives sharp. A loaded strop realigns a rolled apex and removes the last burr, which means most edges never get dull enough to need a stone. For the money it is the highest-value item in a sharpening kit.
Two things. It straightens an apex that has rolled over under load, which is what most dullness actually is. And with polishing compound on it, it removes a burr left by a stone, which is what turns a technically sharp edge into a genuinely keen one.
It removes almost no steel, which is why stropping often is so much better for a knife than sharpening often.
Always stroke edge-trailing: spine leading, edge dragging behind. Stropping edge-first cuts into the leather and rounds your apex. Use light pressure and a consistent angle.
A leather belt, or even the back of a leather notebook cover, works if you do not have a bench strop.
Bare leather realigns. Leather loaded with a fine polishing compound also abrades very slightly, which is what removes the burr and polishes the bevel. Green chromium oxide compound is the usual starting point.
It is also the right way to maintain a convex edge.