Why have a leap year?

Answer 1

Because there are really 365.25 days each year, and since we forget about the extra .25, we add on a day every 4 years.

Not only that, the Earth is slowly decelerating causing the days and thus years to become longer and longer over time. To fix this, we add a leap year. Eventually, we will have to do more than that such as leap days, leap hours, leap minutes, and so on.