Because sometimes understanding the base concept is important. I cannot possibly ask you to solve 1,000 scenarios in 45 minutes. But if you get the basic idea o

Because sometimes understanding the base concept is important. I cannot possibly ask you to solve 1,000 scenarios in 45 minutes. But if you get the basic idea of how the concept works, I can be sure that when you encounter scenario #945, you'll have the basic grasp on the concept to at least know where to look, and when scenario #487 comes up, you'll know the basic idea of how to handle it. Asking you just to do one thing in an interview risks accidentally hitting one of the ten-twenty things you do know how to do, leaving me with no proof that you can solve the other 980-ish possible problems.