That's not a law, that's just some dude who's edited Wikipedia with a couple of numbers pulled out of thin air and called it a law. Compare with Pareto Principle, which is more firmly established in other contexts. I'd like to see what research has been done (if any) on the accuracy of this "law".

But basically the answer to your question is, some code gets executed much much more frequently than other code. Loops are often the reason for this. Other reasons are time-consuming calls e.g. to external resources like web services or storage media.

That's not a rule, that's just some dude who's edited Wikipedia with a couple of numbers pulled out of thin air and called it a rule. Compare with Pareto Principle, which is more firmly established in other contexts. I'd like to see what research has been done (if any) on the accuracy of this "rule".

But basically the answer to your question is, some code gets executed much much more frequently than other code. Loops are often the reason for this. Other reasons are time-consuming calls e.g. to external resources like web services or storage media.