The world is full of hidden mysteries; mysteries that deserve asking the big questions.
Questions like, "Why are we here?" and "What does that springy thing near the base of the door do?"
If you're a cat, the answer to the first question is, of course, "to play with that springy thing near the base of the door," and the answer to the second question is irrelevant, because regardless of its actual function, it sure is fun to play with.
As evidence, we humbly submit this video for your consideration. It's nothing but cats playing with door stoppers for two straight minutes, and it's fantastic.