Uses for tracks in 9648
My FIRST LEGO League team likes to use them as clothing accessories, like belts, headbands, sashes, etc.

More commonly, though, they are used for conveyor belts (for example, in a great ball contraption). You can also use them for a chain drive when 3711 is not strong enough or big enough.

I also saw this recently where small carriages were attached to the links and it was used as a vertical parking system for matchbox cars.