When I sing, I want to be--and am--personally
involved in the communication of the song (most
commonly a song that tells a story), and a big
part of that is the personal eye contact with
listeners, with my being visibly involved in
"telling" the song to listeners.

If the audience is too large to make ACTUAL
eye contact, my eyes run through the audience,
which gives the listeners the feeling that I'm
looking at various other listeners, and gives
me much the same feeling about communicating
the song.

As I see it, singing from a paper or book
(or tablet, I suppose) prevents both of those
functions. YMMV.

Dave Oesterreich