Many black & white prints can be made more interesting and attractive by toning or changing the color of the existing image. It is a means of creating moods and impressions in a picture. We often see older photographs that have a brown or yellowish-brown color and think that this was the way older photographs were made. Unfortunately, in most cases this is not true and the yellow-brown tones are signs of deterioration. These colors are what we perceive to be the natural color of older photographs because we see so many photographs today that are deteriorating.
Actually, true sepia toning* can help stabilize a photograph and make it more archival. Sepia toning converts the metallic silver in a photograph to a sulphide which will significantly increase the life of the photograph. As you can see, there are two major benefits from sepia toning. The new photograph is made to look more like those from the mid to late 19th century while it significantly increases the life of the photograph.
*Some labs make their photographic copies on color photo paper. This enables them to use filtration to simulate the sepia effect. However, this creates two problems as this is not a true sepia process and does not increase the life of the the photograph. In fact, just the opposite is true as color photo paper is far less stable than black & white paper.