I learn most of my tunes by ear, but there are always one or two bits of every tune that are a bit indistinct. The written notes help to clarify the "iffy" bits.
Also if you are ever given a book of tunes/ manuscript of songs from way back when, it enables you to read the lot and work out what tunes/songs were being played/sung a hundred years ago or more. For a researcher that is a BIG bonus.