After over 30 years of performing professionally - I've never been a 'day-jobber' - I've come to realize that my audiences expect a lot of my familiar songs and routines. If I leave out something, a funny intro to a song, perhaps, they'll ask me why. I get requests for comedy routines and songs for heaven's sake, from people who've heard them hundreds of times.
I wondered how someone could sit through a routine to which they know all the punchlines! And one of the answers is: they like to watch other people laugh at the material. They love to sing along with their old familiar favorites.
By the same token, I'm constantly adding new songs and bits because over time they will/may become audience favorites too.
There's a comfort in ritual. Look at Johhny Carson's old Carnak routine, or Art Fern or any of his characters.
The audience knew exactly what was coming but they all laughed anyway.
I saw the Dubliners, The Clancys (with Tommy Makem) and the Corries many times back in the '60's, and they did pretty much the same show each time, with a few new bits thrown in. It's knowing what your audience wants, giving it to them and adding something new to make them want to come back.
I don't use a music stand or song book or a set list.
If I screw up a lyric, I can usually improvise something pretty damn close on the fly.
By a somewhat different token, I've seen opera singers - Pavarotti is a good example singing from a little notebook, and the great Irish tenor John McCormack used to do it all the time at his concerts. He did it in his movies whose names escape me at the moment.
That's OK for them.
But I have a little difficulty with a country, bluegrass or folk performer (alright, let's make that a Full-time professional country, bluegrass or folk performer) using a book or music stand.
Just my taste, I suppose.

Seamus