The Export Material User Data is a translation layer for the Exporter. The exporter doesn't have access to the raw HLSL of the material in UE4, nor do most games or applications have a shader path that allows use of custom shaders. BeamNG.Drive, for example, has a static shader path, and as such, it is a constant struggle to get Automation's paints and materials to export nicely.
The Export Material User Data contains the logic that the exporter can use for adjusting textures and the UVs of the exported meshes, in order to get a close approximation of Automation's materials, for use in other rendering engines.
The layout and structure of the Export Material User Data must be kept in mind at all times during the creation of a Custom Paint or fixture material, as there are certain limitations that cannot be overcome.