Long ago, in the days of roadside tyre repairs and other mechanical issues, tool rolls were standard equipment of every motorcycle. Spare spark plugs, some spanners and other tools, rolled in a rag, you'd better have them with you.
Nowadays, motorcycles hardly ever break down, roads are infinitely better and technical skills aren't as vital as they once were, but fitting a tool roll can still have a visual benefit.
Certain types of motorcycles just look better with a tool roll, and looks are important. Especially when that slightly greasy touch of retro looks is desired.