Well, it's collected, and if they already have it in a dump truck, why wouldn't they re-use it? In the spring, the street cleaner cleans my whole street, and dumps each load at the dead-end. Later, it gets loaded into a dump truck and taken somewhere (3-4 dump truck loads in total, estimated).

The last few years they've used sand a lot more sparsely (only on intersections and hills), so I'm pretty sure the street cleaner only picks up one load and can carry it away without needing a dump truck.