Look a little closer
Leap years keep calendar dates from slowly drifting away from the seasons. They are needed because Earth's seasonal cycle, the tropical year, lasts not exactly 365 days but about 365.2422 days.
A calendar of only 365-day years loses about 0.2422 day, roughly 5 hours and 49 minutes, each year. The difference approaches a full day after four years and would eventually move familiar seasons across calendar dates.
The Julian calendar reduced the mismatch by adding one day every four years, creating an average year of 365.25 days. That average was slightly too long, so the calendar's spring equinox still shifted over many centuries.
The Gregorian calendar introduced in 1582 refined the correction by inserting 97 leap days in every 400 years. Years divisible by four are normally leap years; century years divisible by 100 are not, unless they are also divisible by 400.
That makes 2000 a leap year, while 1900 and 2100 are common years. The resulting average calendar year is 365.2425 days, very close to the seasonal cycle, although not mathematically perfect over unlimited time.
February 29 does not create extra time. It collects the fractions that an integer-day calendar has been leaving uncounted. A leap year is a practical compromise between whole numbered dates and the continuous motion of Earth around the Sun.
