Um, a lot of things?

A computer can give me clues as to why I still feel tired after sleeping for 10 hours every night. The right tracker can tell me what my oxygen levels were while sleeping, or how restless I was during the night. It can estimate how much time I spent in each sleep cycle, and identify anomalies in how much REM sleep I seem to be getting, for example.

Many, many people suffer from various sleep abnormalities and insight into their behavior while sleeping can be helpful for helping their conditions, or at least giving them insight into what's going on.