I shoot stills and videos for display on my UHDTV in 4K. I produce slide shows with small video clips interspaced from a vacation for example. Since the TV and videos are 16:9, I also shoot stills with the camera set to 16:9. I like the whole TV to be filled up. I want the stills to match the video format of 16:9 so I don't get black borders on the stills which would happen with another format.
When I first shot vacation pictures, I would shoot 4:3, the full pixel resolution and then crop to 16:9 in post processing. But often, I missed the ability to crop aesthetically because I might have to crop off feet for example to get the 16:9. So I switched my shooting mode to 16:9 so I can compose in the camera.
However, I still have some ability to change the crop afterwards. Since I shoot Jpeg + RAW, the camera catches the full 4:3 in RAW so there is space on the top and bottom that I could use in post by moving the 16:9 framing up or down. I rarely need to do that however because I usually frame how I want it in the camera. But there are a few shots that do need some adjustment.