Someone once said to me that if you sing from a lyric sheet you will never learn the words by heart. There's a lot of truth in that. I have a friend who sang the same closing song every Saturday night for over 10 years. The song has only two verses yet he wouldn't/couldn't perform it without his lyric sheet in front of him.

"In the old days" we would painstakingly write out each line of a song while listening to it on a record player or tape deck. Writing the lyrics down helps to embed them in your memory. Because we can now just cut-and-paste from the net, that process has gone.   

I should add that as we get older it is becomes harder to memorise lyrics so please allow us elderly folk some slack re. the need for a crib sheet.