2. It Improves Your Reaction Time.
A picture is all about capturing the right moment. There was a time I used to bring my 100mm macro lens, 11mm ultra wide for landscapes and a 50mm for portraits on my photography trips. The problem is when I would put on my ultra wide lens for landscapes; I started to see macro subjects. When I went with the 50mm for portraits, I would see landscape opportunities. At that moment, I have to switch on the correct lens and photograph it. This switching is not only frustrating but I also lose the best moments a lot of times. The same happens with camera bodies.
If you only have one camera with one lens (preferably a prime lens), instead of switching to an appropriate lens, you will capture the moment right away with your current setup. Eventually, you will know how to work around with the current setup in all situations. You can capture a portrait with a wide angle lens or a landscape with the long focal length too.
3. It Makes You Familiar With Your Camera Settings
Different cameras and lenses behave differently on the same settings. The same ISO setting on your full-frame camera produces less noise than your micro four thirds sensor. Different lenses have different sweet spots for sharpness and depth of field. When you use multiple cameras/lenses all the time, you can’t remember these settings and end up regretting that you didn’t use other another combination that would’ve been better suited for the shot.
When you use one camera/lens, you get a lot more familiar with the best values of aperture, ISO and shutter speed settings that will give you the best results for each scenario. You don’t even have to think about it and it comes to you automatically.