The reason it works out so well (contrary to many people's intuition) is that most programming is done in structured languages or in a structured-style these days. This significantly reduces the number of entry points (typically down to 1) for any block of code, though it can still have many exit points. Unless someone abuses something like Duff's device or uses goto's to jump into a block (and the language permits it), the flow diagrams like in the linked image end up being pretty tidy in practice.