Music increases my motivation by making me feel like I am the main character in a story (since I have a "soundtrack") and prevents me from being distracted by people talking nearby (which makes it extremely difficult for me to study) because I can no longer hear them them with sound coming through the headphones.

Also, some music can make me feel more relaxed than I otherwise would (possibly due to sounds that mimic peaceful ancestral environments, such as gently moving water, etc...).