Nobody plans an event thinking about copyright law. They think about the band, the room, the catering, and the guest list. Music licensing sits in a blind spot precisely because it is invisible when it works, and it works almost all the time, because the venue quietly handled it years ago and never mentioned it.
The trouble starts when the event moves outside a normal venue. A ticketed show in a rented warehouse. A festival in a public park. A big company party on a corporate campus. A charity gala in a space that has never hosted live music before. In all of those cases the assumption that somebody else has the license is doing a lot of unexamined work, and the person holding the risk is usually the event organizer, not the band.
The second problem is the confident misinformation. People will tell you a license is not needed because the band is playing covers and covers are fine, or because the event is free, or because it is for charity, or because it is only a few songs. Those beliefs are mostly wrong. Playing covers is exactly the situation a performance license exists for, since the songwriter of the covered song is the person being paid. Free admission does not by itself remove the requirement. The statutory exemption for nonprofit performances exists but is narrow and has conditions, and it is not a general blanket for anything with a good cause attached to it.

