Sand is one of the ultimate stages in the disintegration of rocks.
In deserts, the principal agent of erosion and the transport of materials is the wind (wind erosion). If the wind can act so effectively in eroding and in the transporting of particles, it is because there is neither humidity nor vegetation nor relief to hold and stabiilze those particles. The wind that sweeps the surface of the ground thus easily carries them away.
A sand dune can, via erosion, itself move from its windward side and accumulate on the opposite side. The landscape is thus constantly renewed.