When friend’s performs live with cheering crowds screaming for them it is hard to hear the music or the band. As a group they needed to hear each other crystal clear or else they might be ruining the show and you can’t blame the crowd for screaming because that’s also your fault; that is what you wanted them to do and it wouldn’t be good if they stay quiet. You wanted them to enjoy the show as much as you wanted to give them the best performance without any audio flaw.
This is why having an in-ear wireless monitor comes handy for those bands out there not just in bands but in every theatrical performances that deals with big crowds. This technology helped thousands of performers out there not to mess up on stage and just giving the crowd what they wanted.