Some singers who accompany themselves on guitar make a point of performing without a 'crutch' (some may have crib sheets stuck to their guitars).
Some memorise songs and can perform them beautifully, but if you've seen them before you may know that their 'set' will perm any three of the eight songs you've heard them sing before.
Others have learnt lots of songs, and can sing any of them 'at the drop of a hat' (good title for a revue, that).
Some supplement their supply of memorised songs with paper versions, which allows them to try something new, or fit in with a themed evening (at short notice).
Yet others only sing from paper. They may improve, and move into one of the earlier descriptions, but probably not if they are censured for singing from a sheet.

Diff'rent strokes for diff'rent folks