Google Fit's open APIs allow any connected apps and devices to seamlessly share data, all of which gets sucked up by Google Fit.
The idea is that we've reached a point where basic doctors' visits have become more or less obsolete. While that logic is debatable, we are perfectly capable of measuring all our vitals. And Google wants to give us the tools to do it. As BussinessWeek wrote in a profile of Sundar Pichai, the chief of Google's Android division:
Pichai says it's "crazy" that people visit their doctors, at most, once a year to have their heart rate and blood pressure measured. "You obviously need to be able to measure these things so many more times and then apply more intelligence to it," he says.