Very simple- my instructors told me it was good, so I listened.
Eventually I learned that selenium basically attaches itself to the silver, therefore more action in the shadows.
I use a strong selenium 1:5 with different short times.
Now I use sepia for the highlights and selenium for the shadows. According to Tim Rudman this is a very archival method of toning prints, and it looks very beautiful.
Tri toning with sepia, gold then selenium adds a pop to prints and colour that I find very appealing.
Selenium is not required, just a aesthetic step that many of us use , and we all use it differently.
I have a silver print hanging in my darkroom that I gave too my father when I graduated from photo school.
It was on Kodak Ektalure and heavily selenium toned and it still looks great.
there are chemists here who can speak to the archival aspects of selenium but I use it mostly for the look.
Bob, what was the initial reason?
When I started in 1991 or so, Se was optional, now it seem a must for the job.