Depending on the media choice used, Selenium toning can result in images with purplish brown tones.
The use of a selenium toner increases print permanence.
Selenium toning was originally devised to increase print permanence and reduce fading.
Even today, selenium toning is a sensible precaution for prints that are on continuous display.

Tech talk
Prints toned generally have higher density and contrast than untoned prints. They also have a higher maximum density.

Aesthetically
The look and feel

Lloyd Erlick – What I have Learned about Selenium Toner
http://www.heylloyd.com/technicl/seuse.htm