The word “band” may include several aspects. It defines a group of people playing music. It is also called a musical ensemble.
In sociology, groups of people unite for specific reasons, and people within a group have specific relationships resulting in them gathering.

In music, and it is obviously the major field in which “band” is used, it designates a group of people who play music. Even before Mozart's time, groups of musicians – then called a musical ensemble – formed quartets or trios, and so on. The instruments which were mainly used were the violin, the cello, and the viola. These were called string quartets or trios.

However, the meaning evolved during the 20th century and especially in the beginning, when jazz bands appeared. Towards the end of the century, jazz bands and rock bands were common.

And then, there were boy bands... These groups were made up of … boys obviously, who sang repetitive melodies and danced some choreographies following the rhythm of the music (or trying to). Most of these bands were made up of boys who had no particular artistic skills, but who were rather good looking. They peaked in the 90s.

Being in band, being part of a band makes people more confident and seems less frightening than being alone. Therefore, even if most of the time, the singer is the leader of a band, musicians, usually, feel OK about it.