This grip gives the piled sand some tensile strength, which can be exploited to make deep undercuts, leaning towers, long thin structures and counterpoised elements. Not all sculptures call for such elements, and other, less labor-intensive packing methods are good enough. So, for big blocky bases and anything else that just has to sit there, packing damp sand with your hands is good enough. This sort of pile will even accept some carved detail, but don't ask for much. In particular, don't ask for big vertical surfaces.
More detailed structures require better compaction, such as slurry-packing.
This is an art in itself, the piles lending themselves to flowing, spread-out shapes in a sort of Art Nouveau style. It can be done with no more tools than your hands and some decent sand, if you're willing to sacrifice your fingernails. There are limits on size, but with creative use slurry-packing for details, big sculptures are possible. Just ask Sandy Feet. If you want big, well compacted piles, you pretty much have to use a form.