When i was fully engaged in a band group, I learnt that if you are a good keyboardist or vocalist who does not cooperate with the drummer or other instrumentalist, the performance at the end of the day would be a mess but when every works together to cover each others track, the performance is awesome.
Also working alone as a solo vocalist may sound ok, but when there are two good vocalist work as a team, it bring out sweeter melody because the two will be singing different parts at the same time; adding more color to the song