Here's why:

* Wind Action: Barchan dunes are created by wind carrying sand grains. The wind, typically blowing in a consistent direction, erodes sand from the upwind side of the dune.

* Sand Movement: As the wind carries the sand grains, it deposits them on the downwind side of the dune, creating the crescent shape.

* Slipface Formation: The downwind side of the dune is known as the "slipface". The sand grains accumulate and, due to gravity, gradually build up until they become unstable. This leads to a steep slipface, where the sand avalanches downwards, further contributing to the crescent shape.

In summary, while erosion plays a role in removing sand from the upwind side, the defining feature of a barchan dune is the deposition of sand on the downwind side, forming the distinctive crescent shape.