It turns out that the fake speaker grille on the bottom left of the iPhone 7 and iPhone 7 Plus does serve a true purpose.
Apple told The Verge that the space formerly occupied by the 3.5mm headphone jack helps the handset’s barometer function correctly:
according to Apple it’s a “barometric vent.” Apparently adding all the waterproofing to the iPhone 7 and 7 Plus meant that it was more of a sealed box, and so to be able to have an accurate and working barometer, Apple used that space. The barometer is the thing that allows a phone to measure altitude, and Apple points out that on the iPhone 7 it can measure even minor changes like climbing a flight of stairs.
You can read more about iFixit’s teardown here. Some of the more interesting tidbits include a larger battery and 3GB of RAM on the iPhone 7 Plus.