Why is Leap Year necessary?
Leap Year is necessary because our clocks (based on Earth’s rotation on its own axis) and our calendars (based on Earth’s revolution around the sun) are incommensurate, based on unrelated cycles. We need to fudge one system or the other every now and then to keep them synchronized. Leap Year is just the system we have historically adopted. Actually, even the Leap Year needs tweaks of its own. This video discusses the first, second, and third order corrections to make clocks and calendars agree as precisely as possible!