It's evidence to show people (family, doctors, etc... and yourself). It's harder evidence than flaky human memory or "impressions" of how I slept, in any event. You can literally see how crappy your sleep is, on a chart. It's rather eye-opening, and if you have a wife or girlfriend who wonders why you're a space cadet today who can't get anything done, you can always point to the "4 hours of good shuteye" chart from last night (if it exists) lol