The most persistent rumor about the iPhone 7 has been the removal of the headphone jack. There has been a lot of hostility and ink spilled on this topic like Nilay Patel's "Taking the headphone jack off phones is user-hostile and stupid". And there has been lots of speculation as to why Apple is making this move, including an effort to make the phone thinner, more water proof, or just because this is how Apple rolls (floppy disk, CD, etc.).
However, I think there is one reason for this rumored move by Apple rarely mentioned, and that is to drive the adoption of wireless headphones that will work with Apple Watch.
Whereas the strongest rumor about the phone has been about the headphone jack, the strongest rumor about the next Apple Watch has been the inclusion of GPS. Secondary rumors are the inclusion of a barometer, superior waterproofing, and higher capacity battery (see MacRumors article). This is all about making Apple Watch more of a fitness device. A greater ubiquity of wireless headphones will increase the appeal of the Apple Watch as a stand-alone fitness device. You will no longer need to run, ride, or workout with a giant phone attached to your arm.
I honestly believe that Apple sees widespread adoption of Apple Watch as one of the most powerful ways that Apple can improves lives for the better. You see this in companies they have acquired like Gliimpse (The Verge article) and in libraries that Apple provides developers like HealthKit, ResearchKit, and CareKit. In 2015, when Jim Cramer asked Tim Cook about next frontiers, Cook said heath was "the biggest one of all" (MacRumors article).
A thinner phone, a more waterproof phone, edge-to-edge OLED screens - these are tweeks to an already incredible device. They won't fundamentally change your life.
It is improved health, with Apple Watch spearheading that effort, that is a driving passion at Apple, or at least with Tim Cook. And I believe that the push for wireless headphones that work with Apple Watch is just one small but important part of Apple's larger health strategy.