There are a few reasons you might want to add a stair runner to your staircase.

• Stair runners are a good way to protect your stair treads from wear.  After all it is easier to replace a stair runner than stair treads.
• Stair runners are safer particularly when coming down a staircase.  The runner provides a better gripping surface than a polyurethane finished stair tread.   
• Stair runners are nice looking (provided you purchase an attractive one) and will give your staircase an extra special look.

This Old House has quite a nice article with step-by-step instructions for the do-it-yourself type.  If that is your cup of tea you will like reading this piece at This Old House