For decades the 35mm format with its 3:2 aspect ratio has dominated the photographic world as the most common format, but as W Eugene Smith once said, “The world does not fit conveniently into the format of a 35mm camera.”
No indeed, for myself and many others, the letter box aspect ratio of 35mm of 3:2 is a little too long or not wide enough (or vice vera) if you see what we mean? Put another way the frame of a format with the 3:2 aspect ratio is too long compared to those with a 4:3 ratio especially for nature and landscape work (as above) leading you to having to try to find something to fill the extra length, which is not always possible or causing you to unbalance your composition.
This becomes even mores dramatic when you turn your camera on its side and view the world in portrait mode i.e. in a vertical format. Thus we prefer a squarer format and find an aspect ratio of 4:3 to be ideal. In my case my Panasonic Lumix TZ95 provides this, but there are many other cameras in many other formats that have this ideal aspect ratio from micro four thirds and up to medium format. Or you can always crop your image in post processing I guess though be aware, as Bryan Peterson said, “There is no better time to crop a bad composition than just before you press the shutter release”.
It's all about framing and achieving the right balance of the elements contained within and bounded by the edges of the frame. Now the simplest and quickest way to achieve this is by having a camera that provides you with a frame of the right aspect ratio i.e 4:3 in native mode in the first place.
In modern digital cameras many that have a native 3:2 aspect ratio will allow you to alter this in camera to say 1:1, 4:3 or 16:19, but be aware they achieve this by cropping the image and if you save it as a jpeg you end up cropping its "resolution" too (a RAW image will always be saved un-cropped so be aware of this too and thus you will have to re-crop it in post processing from memory).
Ironically a digital camera with a native 4:3 aspect ratio will typically let you set it to make 3:2 ratio image as well, but this too is achieved by cropping the image (in jpegs) which seems to defeat the object!
Well, if the world does not fit conveniently into the format of a 35mm camera i.e the 3:2 aspect ratio will it always fit conveniently into one with a 4:3 ratio. Well in my experience it will do so much more often than not but... there are times when an even squarer format like 5:4 or even totally square at 1:1 are to be preferred. Indeed at the other end you may want to go to a 16:9 or a longer frame for panoramas, so be aware norms are there to be ignored, they are just a set of guidelines really.