Before people panic over Apple's obsession with thinness, realize removing the 3.5mm jack would save some internal space regardless of how thin the iPhone 7 will be. That means that the iPhone 7 could have the same thickness as the 6 while having a larger battery due to internal space saved.
Two components would be removed by this move, the relatively large
3.5mm jack module and the DAC (digital to analog converter), which would now be integrated in headphones rather than inside your phone. The DAC is one of the largest component on the PCB right now, I believe only the SoC, NAND and LTE modem are larger.
Personally, I don't think the iPhone 7 should be thinner than the 6, but I would still appreciate if the 3.5mm jack got removed.
On top of the saved internal space, moving to Lightning provides certain benefits:
- Possibility of using a DAC that's better than the one in the iPhone in high-end headphones.
- Less degradation due to the transmission through an analog cable. Also less likely to hear a hiss when moving the jack due to dust etc.
- Possibility of headphones recharging your phone or vice-versa. Particularly useful for docks and soundboxes that both charge your phone and play music using a single cable.
- Possibility of sending additional data from your headphones to your phone. Stuff like battery level in noise-cancelling headphones. Or increased reliability and features of headphone remotes.
Edit: Scratch that about the DAC removal part. As some have pointed, you still need it to use internal speakers. Apple may choose a lower-quality internal DAC that's significantly smaller though, since you don't need a good DAC just for speakers.