We use data with the hope of changing our behavior. We need to keep records of this data because our memories are not trustworthy. In regards to my weight, it is only due to records that I've kept for the past number of years that I know how far I've come in the past ten months (although I'm on a reverse trend at the moment).
I heard a speaker once say (at a conference on data management in education) that change cannot happen without data. I don't necessarily go that far, but I've seen the power that data can provide ... for evil as well as good. For I've also seen people misuse data to serve their purposes, often to mislead the public for the sake of their own personal profit. So often, if people just had a better understanding of how data works, it could not be used in these ways against them.
There can also be too much data. An overload of facts and statistics often dulls our attention to things which are important. We don't want to think about it because we are happier in our ignorance. But ignorance does not solve problems and certainly doesn't make the world a better place. We need to learn to manage as well as understand the data around us.