Take the musical genre bluegrass, for instance. As separate words, "blue grass" can mean the literal plant grass that is the literal color blue, so you need to stick the two words together to differentiate it. Same with back slash (backwards cut with a weapon, slash someone's or something's back, etc.) vs. backslash (the specific symbol "\"), black bird (a raven or crow) vs. blackbird (a specific species), copy right (correctly reproduce something, the privilege to copy something, etc.) vs. copyright (the specific legal system covering copying privileges and fees), home work (painting your house, working from home, etc.) vs. homework (assignments a student is given to complete outside of school), and so forth.