Varnishing provides a more or less homogeneous surface. Picasso and Braque were after something else, a contrast of textures in contiguous areas of the painting which would enliven the surface and convey a new sense of depth. ...
[The restorer Michael] Duffy, a boyish-looking man who appears unfazed by his responsibilities, walked up close to the painting and indicated the vigorous, cascading brushstrokes that are now visible. “It’s that sense of somebody touching the surface that I think is greatly retrieved,” he said. Duffy dreamed recently that he came into the studio and saw that a corner of the picture had peeled back. “It was as if the figures were trying to come to life.”
(The New Yorker never fails to tell us what everybody looks like. What the hell difference does it make if Duffy's "boyish-looking"?)
Tomkins notes that Picasso did not varnish his own paintings. But that point should be made much more strongly. Picasso hated varnish. I can't locate this anecdote on the web, but I read it long ago and it made a great impression on me. Someone had acquired a painting by Cezanne, and everyone was raving about how wonderful it was. Picasso would barely even look at it and was quite disgusted. His angry comment was "Where is the hand of great Cezanne?" Varnishing had destroyed the painting. You could no longer see the brushstrokes, the life of the thing. And today, when you go to the museum, most of the paintings are underneath layers of varnish, giving them a vile, shiny surface that is not what was intended. You may as well stay home and look at photographs of some of these things, because the works themselves have been laminated. It doesn't matter so much in the case of paintings of artists who tried to create a slick surface and used varnish themselves to smooth over the brushwork, but even then, it is bad for there to be any more smoothing and slicking than the artist chose for the painting. Varnishing is really horrifically ugly.