Desktop originally refers literally to the surface of a desk. The noun has been adopted as an adjective to distinguish among office appliances which can be fitted on top of a desk, as contrasts with a larger piece of equipment which covers its own area on the floor. Examples are:
A Desktop photocopier, a smaller photocopier for individual use, compared with the professional models.
A Desktop computer, a personal computer different from that in a "tower" computer case designed to sit on the floor, and also different from a portable laptop or "notebook computer".