If you regularly type unusual brand or product names, technical terms, foreign words, or other “niche” terms, then at best, your Mac will keep underlining these words, but at worst it’ll step in and attempt to autocorrect these “mistakes.” Your Mac’s built-in spellchecker can be an invaluable tool for catching spelling mistakes and typos, but if you regularly type words that aren’t in your Mac’s dictionary, then macOS’ spellchecking functionality can actually make typing more difficult.
