I sometimes see a natural where it is unnecessary. Sometimes it is a courtesy natural but other times the note has not been flattened or sharpened at all and yet I see a natural that is already there because of the key signature.
Sometimes I see a flat on a note that is already flat. The most common note I see this on is Bb. Like there might not be any B naturals and yet I see a flat on a Bb. This could cause quite a bit of confusion. Some people might think it must be Bbb or A whereas some might think that it is still Bb.
A similar thing is true for a sharp on a note that is already sharp. Some people might think it must be Fx or G whereas some might think that it is still F#.
So why do people put naturals where they are completely unnecessary, flats on already flat notes, and sharps on already sharp notes?