The vast majority of our medical bills, these days, are for chronic conditions lumped under the term "diseases of civilization". Obesity, diabetes, heart disease, acid reflux, hypertension, etc.
Underlying all of these is systemic inflammation. Reducing inflammation improves all of these. But measuring inflammation is relatively expensive, involving trips to the doctor, blood draws, etc.
If we had a device that could measure inflammation as quickly, easily, and cheaply as the over-the-counter glucose meters, people could monitor their own inflammation levels and determine what dietary and lifestyle choices had what effect, on them.
There's a good possibility that we could build a device to make exactly these measurements, by examining the waveform produced by a simple photoplethysmograph sensor.