It is less about "knowing his material" and more about recovering from distractions.
Having toured with a number of well known, and very talented performers the
teleprompter is used not because they don't know the show, but because it is so easy to be distracted.
Imagine being on the 150th show of a busy tour, and while you are singing your hit song you get a little distracted... A camera flash, or a
laser pointer, or a clever sign an audience member is holding... then you think..." crap... is this the second or third chorus?... I think it is the second...." look down at the
teleprompter, and see oops... yup, you are in the third chorus. song saved, and the audience will never know.
It is a useful tool for giving the crowd a great show every night, and laypeople would be suprised at how many great performers use one.