[adrianratnapala]

It's intended to be restrictive. The idea is that more than three levels of indentation should be the exception rather than the rule. Exceeding that without good reason is a hint that functions should be split up.

Having some such rough limit is a good thing, but the number 3 is language dependent. Java for example automatically eats one level for the class.

Also C lacks syntax such as nested functions, try blocks, python-style context managers and all kinds of other stuff which excuse more levels of indentation.