Friends/relationships, for checking general activity and check-ins.
Potential businesses, for seeing interests and the like. Or, if currently employed, the employer could randomly check to see how the employee has been sleeping (are they going to be tired/cranky today?) or to see how often they check-in at the office.
Aggregated with more data from people doing similar things and you can start to get a look at populations (what if everyone in SF was logging this data? What if a good chunk of those in the US were?).
The quantified self people all claim if you can't measure something, you can't improve it. It is very popular with runners. If you don't track your time or run competitively, you tend not to improve your times. Runners are willing to put a lot of data in, even like how they felt after each run, which has good uses for planning work outs, etc.. But the same concept applies to anything - eating healthy/less, walking more, smoking less, etc.. Like good old Ben Franklin used to track virtues until he mastered them.
