Answer

Think of the function that we wrote a few lessons back to approximate the square root of a number. That program consisted of a loop that computed successively better approximations to the square root of some number. In that case, we want to stop approximating when we get an answer that is close enough to the true square root to suit us. And we can't tell in advance how many iterations that might take.
Imagine writing a function that takes an array and a number as arguments, and returns an index into the array where the number is stored. The best way to implement this is by using a loop to iterate through the array until the desired element is found. If we are using a plain DO loop, we'll have to continue with the loop even if the desired number is found in the first iteration. We'd prefer to stop the loop as soon as the desired number is found.