These nodes can be used to retrieve custom user attributes from a geometry (e.g. Display Color) or read custom particle channels, etc. See the User Data tutorial for more details.
The available built-in user attributes are listed in the respective user_data shaders.
It is also possible to read user data fields from volumetric shapes, allowing things like per-particle user data on volumetric spherical point clouds to affect the result of volumetric shading.