Baryta is a naturally occurring mineral technically known as barium sulfate. This snow white clay is used as a white pigment ingredient for paint. It is used as a brighter for art papers. Photo papers are coated with baryta to get increased whiteness and a higher reflectivity.
We view photo prints by reflected light from a nearby light source. Light plays on the print and transverses the dyes and pigments that comprise the image. This light then hits the baryta undercoat and is reflected back towards the viewer. Thus the illuminating light makes two transits thru the image on its way to the viewer.
The result is an enriched presentation of the artwork. The sheen and whiteness gives a pure white that keys off the tones and colors. The result is an increased perception of contrast and sharpness. The baryta coat enhances most artwork.