Thinking about the future can often trigger morbid thoughts, but there's something powerful about seeing how much life you might have left. The death clock helps with this process of reflection, projecting your possible demise based on some basic lifestyle factors.
It can be inspirational if you take it the right way. I guess this may be along the same lines as the aging machine I linked to below -- some people will find it fascinating and cool while others won't want anything to do with the unpleasantries of envisioning their old age. Two other life clocks (this one and this one) take a more positive approach, use more lifestyle factors, and both gave me an extra three or four years to live, so I should really recommend them as well.