So why bother with toning? Sepia and Selenium toning were originally used to preserve printed photos and make them last longer. In fact, it is not uncommon to see sepia-toned photos from seventy, ninety or even a hundred years ago in great condition.
This method was so common in the past and there are many photos that survived to our times. This is why we have a strong association between sepia tone and old times, vintage, nostalgic, old school looks.
For certain types of digital black & white images I find this technique adds an almost 3D effect and brings back that darkroom feeling.