Trad music is very difficult, if not impossible to notate as played. For example, changes in bow pressure, subtleties of phrasing, ornaments, etc. There is standard notation for bow direction, but it?s rarely used for folk music. As with any style of contemporary folk music, and with early classical music, the written sources are nothing more than a rough indication of what actually gets played.

Ear LEARNING makes you a better player. Every player approaches a tune differently, and each repetition of the tune should aim to be unique. Learning by ear helps you become more attuned to these differences, and makes your own playing more varied and interesting. When you learn a tune by ear, the tune seems to enter a different part of your brain?the part that?s directly connected to sound and music.  Though reading music is a very useful skill, when you stare at a piece of paper while you play you?re taxing your brain, making it do visual processing, instead of aural processing. For some people the visual processing makes it almost impossible for them to do some or all of the following: listen to what you are playing, to listen to what others are playing,  pay attention to how you are handling your instrument, be cognizant of your body, draw the rhythm into your body.  When you play your eyes should be used to make contact other musicians or the audience. Staring at the dots on the page makes you oblivious to what is going on around you ? just like walking and texting, or worse driving  and texting.

Source: Learning by Ear | Slowplayers.org