Why open-source matters: I can remove annoyances like starting in front-facing mode from a smartphone camera software, and hide the button for the "effects" drawer that I never use since I can add Sepia or black/white in post processing should I ever need it.

Both of these annoyances cause missing moments. If the source code of the camera software is open, and if the operating system is rooted, these utter annoyances can be removed.

There are open-source third party applications like "Open Camera", but they lack quick launch support and might have, presumably due to lack of optimization, a two-second shutter lag. Big no.