Global properties are used if a shader needs them but the material does
not have them defined (for example, if the shader does not expose them
in Properties block).
Usually this is used if you have a set of custom shaders that all use
the same "global" vector (for example, wind direction). Then you can
set the global property from script and don't have to setup the same vector
in all materials.
Additional resources: SetGlobalFloat, SetGlobalColor, SetGlobalTexture; Material class, ShaderLab documentation.