They are there so that you can think about your algorithm.
For example if you wanted to test if a number was prime by testing if it was divisible by any integers less than itself. This would be ok if all of the numbers you wanted to check divisibility for were less than 1000. In this case the numbers can be up to 10^9, this algorithm would take a very long time to find that ony one number was prime. We need to look at many.
