Under the pictorialist influence, varnishing was done in the early 20th c to add brillianace to matte paper. Normally a lithographer's varnish was used. It became unnecessary as papers with a greater brilliance became available.
At the time, older workers remembered how albumen prints dazzled the eye, and wanted to achieve similar results from platinum and silver. Paul Strand used varnish for the same reason some are fascinated by ferrotyping, to make the details more vibrant in the print.
Today, it is despised by curators for it darkens the print considerably over time.
The technique has little to offer today, for papers are very different than 100 yeas ago. I don't think it adds anything to the appearance of the image, and unless the craftsman is skillful in using varnish, lacquer, or shellac, the image will be diminished.
I don't like putting glass over my silver prints; glass adds the same 'gloss' of a varnish. While it intensifies the overall 'blackness', it also obscures the distinct and delicate dark tones possible with any good silver paper.
d