I was in a band that used these kind of things as stage monitors back in '81! Just vocals (we had backline)...
Nobody sang off key, nobody ever complained they couldn't hear themselves sing, and we could pump up the vocals out front to almost ridiculous levels without a trace of feedback! We also had much longer hair, back in the day! Most of the time, you couldn't even see we were wearing them..!
I still keep a set of these kind of things in my cable case, for the odd gig where I can't set up my hotspot Mackies for monitoring and the PA is ahead of my playing position (or I'm jacked into a distributed house audio).