Store information that is naturally associated with a given material or texture in an external file. Right now this allows to define things like footsteps, toxic ground (hurts player), and bump mapping.
In the future, it should be possible to express all these properties in pure VRML/X3D (inside Appearance / Material / ImageTexture nodes). Right now, you can do this with bump mapping, see http://castle-engine.sourceforge.net/x3d_extensions.php#section_ext_bump_mapping , but not footsteps or toxic ground. In the future it should also be possible to express these properties in 3D authoring software (like Blender), and easily export them to appropriate VRML/X3D nodes. For now, this TMaterialProperty allows us to easily customize materials in a way that is not possible in Blender.
Using an external file for material properties has also long-term advantages: it can be shared across many 3D models, for example you can define footsteps sound for all grounds using the grass.png textures, in all levels, at once.
Texture basename to associate this property will all appearances using given texture. For now, this is the only way to associate property, but more are possible in the future (like MaterialNodeName).