Because the parent rock weather differently in a wet or dry regime. E.g., feldspars are more readily preserved in drier climates, so arkoses form more readily. A temporal change in precipitation patterns will be detectable a mineralogical change in a stratigraphic sequence. The uplift of a mountain range that sets off a monsoonal cycle will be detectable in this way. Gravity has the upper hand.
Disintegrated material is moved to lower levels, leaving fresh bedrock exposed.
This is the kind of sediment transport performed by rivers and some surface and deep-sea currents. Much sediment is transported in the medium of water, either in streams and rivers or by ocean currents. Once they are in motion, sediment particles can move in a variety of ways. Grains slide or roll over the substrate. Grains hop or bounce along the substrate.