Imagine you’re designing a lift (elevator) for a building.

• Sometimes only 1 person uses it (easy).

• Sometimes 20 people rush in together (heavy).
  If you want to guarantee safety, you don’t design for the “average” or “best” case.
  You design for the worst possible load.

Similarly, in algorithms, we want to know the maximum time it can ever take, so that no matter what input comes, the program won’t surprise or fail.