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You may or may not know about Camera2 API but it is actually very essential in making your phone last longer through custom Camera apps and custom ROMs.
Camera2 API, from what I've seen (correct me if I'm wrong), allows for better control of the camera by apps which in turn may allow for better photo quality through third party camera apps and even custom ROMs which, most of the time, have worse camera quality than stock.
Recently, someone has ported the Google Camera with HDR+ to any devices with a Snapdragon 820 and up devices which should include the Axon 7. Unfortunately, Camera2 API being missing, this app does not work with the Axon 7 on stock nor on custom ROMs.
Why does it matter? In short, HDR+ allows much better photo quality during low light situations even on cameras that aren't even the best on the market. The Google Pixel which has without doubt one of the best if not the best photo quality in smartphone doesn't have a particularly good camera but HDR+ allows for much better image quality.
TL:DR Camera2 API is essential in ameliorating the low-light image quality or image quality as a whole. While the Axon 7 already has a good camera, Camera2 API will allow for much better image processing and thus much better image quality.
Edit: Comparison images
OP3T Day (1st is stock, 2nd is HDR+): https://imgur.com/gallery/mEA6p
Xiaomi mi5s collection (which is reputed for having terrible image quality): http://imgur.com/a/aiXbF
I'll add more images but the Xiaomi mi5s album is probably the most staggering difference in image quality.
LG V20: http://imgur.com/a/ATXRC
OnePlus 5: https://imgur.com/gallery/blVXA
The difference is amazing on the OP5 and LG V20.