Let's assume you are writing a program in which user can input the amount of meat they have, and you give them what recipe they can cook. Of course, the user should not give you a negative number. But maybe they do (perhaps by mistake, or on purpose for QA). You shouldn't let them since the program will show "Sorry, we don't have any recipe for your amount of food" instead of "The amount can not be negative".
So, most beginners use "if" statements here:
if input less than 0, ask the user to give you another number
But there is a problem here! let's assume the user gives you -1. Then the program will ask them to input another number. What if they give another negative number (again, for QA)? The if statement already passed and will not run again. So, the problem is not solved.
We should use "while" instead:
while input is less than 0, ask the user to give you another number
The user will stuck. They have no other choice than to input numbers as much as they give a positive number.