Surely you also find this information out very quickly which begs the question why would you need a sleep tracker for longer than say, 2 weeks?
Besides that, I noticed that many people actually don't really know anymore how to live "normal" (aka healthy) and really need those Gadgets. For Example: Make Sports, Sleep and eat well ... that's nothing new, but many people seem to forgot how to do so and need Apps to remind them on a hourly basis to do so (don't forget to stand up for 5 minutes every hour).
Beside all that: Proper sleep is not about tracking sleep and get a reporting that you have slept properly. There are several things that contribute to a proper sleep, like sleeping habits, eat well (and at the right time) and more.