Most bands that take their job and passion seriously include a bass guitar and the reason behind that is simple: a bass guitar is needed to provide both the rhythmic and harmonic foundation. A bassist will support the rest of the band by providing a steady pulse, otherwise known as the rhythmic foundation.
Music pieces and band performances that are appreciated have a steady beat or rhythm and here is where the bass guitar plays a significant role. The drums will also aid in providing this rhythm. Even if every instrument and player in the band should have a good rhythm independently, the bass and drummer lay down this foundation.
Furthermore, the bass will also lay the harmonic foundation. By supplying the right foundational notes, a bass player will define the harmony; therefore, the role of a bass guitar in a band is to lay the foundation for rhythm and harmony.