Why is there a Leap Year?

I am learning why there is a leap year.  A common  year has 365 days and a leap year 366 days, with the extra, or intercalary, day designated as February 29.  A   leap year occurs every four  years to help synchronize the calendar year with the solar year, or the length of time it takes the earth to complete its orbit about the sun, which is about 365¼ days.