The main reason is having metals and non-metals in the same texture and you want the blending to look nice. Also consider cases where the material properties change with a higher frequency than the texture resolution can capture (e.g. metal-dust on a dielectric). If you don't blend this in the texture it might produce aliasing. There are also cases where you might want to use a non-binary metalness for dusty metals or rusty metals or painted metals and stuff like that.

Metalness is essentially F0, the base reflectivity of a material (looking straight at it), which can be high for materials with very high indices of refraction, like Diamonds and other gemstones. Afaik Diamond has a metalness of about 0.2.