Why should camera mounted flash only be used when really necessary?

Camera mounted flash will light the subject from the same angle and direction, leading to a dull and flat image. Instead of mounting it on your camera, if its detachable, take it off and hold it to the side so the image has dimension and comes out way more pleasing to look at. This is why when you go to professional photography studios you see those giant flashes with the hoods (which better direct the light if use properly, using it incorrectly will cause it to cast a shadow over the lens), it creates a better and more pleasing image. I don’t think there’s any instance where any photographer will recommended using camera mounted flash, unless you really know how to use it. I have a Nikon D70 (its a relatively old camera, it came out in 2004 and has a viewfinder still and is pretty manual) and it took me months(almost a year) to learn how to use a camera mounted flash, but i also spent hours each day working with it, which if you dont have that level of dedication or motivation (or just dont have the time) it may take you much longer to learn, which is why i recommend not using it unless you’re really committed to learning how to use it, if not you’re just gonna waste those opportunities for gorgeous pictures trying to use the camera mounted flash without experience with it or without complete understanding of how it will affect the image.

Well, the four major ones to use would be contrast, proximity, alignment, and repetition.