In a band each member has a role when playing music to ensure the music is played correctly to make it sound perfect. different instruments have different purposes for each song and its the musicians job to understand it.
Bass: The bassists Job has a very important job in music to keep in time with the rhythm as the bass becomes the life line and pulse to the music and really reinforces it. As well as rhythm the bassist also has an important job to highlight the tone of the music. The bassist works very close with the drummer to establish the rhythm of the music. Without bass in a song you can miss a key element making the song feel empty and dull.
Piano/keyboard: The primary role of a keyboard/piano player in band is to provide harmonic accompaniment to the melody of the song, But for this to be done right the player must have an in depth understanding of the genre of the music and of chords, to understand how to play the correct voicings or inversions. This really helps the harmony support the melody of the song. The keyboard must work in close associating to the vocalist and focus on the rhythm of the song.
Drums: The drummer is the very foundation of the band as they keep the time of the music as they create beat. Without the drummer there would be no beat and it would be very hard to keep in time. The drummer must be confident in keeping a steady beat and tempo or the whole band will fall pieces. Other instruments such as the bass and guitar focus on the drummer but the drummers job is to keep everyone in check including themselves.
Vocals: The vocalists job is to sing the songs melody which is accompanied by the rest of the band who focus on reinforcing or creating the melody and rhythm. The singer must pay close attention to the music to keep in time and not to miss their queues. A singer can commonly be accompanied either by a guitar or a piano, in that case they would he working in close association with them.
Guitar: The lead guitarists job is to create a melody that fits with the style and tempo of the bass player and drummer, a guitarist may play solos most commonly in rock music. The guitarist also has to be able to play parts or maybe the all of the rhythmic pulse of the song. The guitarist has to work closely to the bassist and the drummer as they have to focus on the rhythm of the song.