In almost all musical forms, the bass has two important functions to fulfill. First, the bass defines the chords being played and guides the movement of the music from one chord to another. This role is usually shared with a guitar or a piano. Second, the bass provides the rhythm of the music being played. This role is usually shared with the drums. Because it links the two functions of rhythm and harmony, the bass is often the instrument around which the rest of the music is organized.
via Bass Lesson. Part 2 – The Role of the Bass Line | Lessons @ Ultimate-Guitar.Com