What is toner for in photography?

Answer 1

It is used to replace some of the developed silver in a black and white photograph with an inert dye. This increased the archival quality of the print and could also be employed to increase the D-Max (maximum density) of the print. When used for those purposes, the toner chosen was usually Selenium on cool tone papers, as warm tone papers could produce brown to blue-black to purple tones, depending on the paper tone.

To achieve a brown tone somewhat like an old Daguerreotype, Sepia toner would be used. In the former case, print exposure often had to be curtailed slightly to prevent the toned print from becoming too dark, whereas brown toning usually required slightly more exposure in the initial print.