It's a leap year which means there's an extra day in the calendar, 29 February 2020. Why we need it?

It's a leap year, which means there's an extra day in the calendar, 29 February 2020, But why do we need it?

It's a leap year,which means there's an extra day in the calendar,29 February 2020,But why do we need it?,sharewhatweknow.com,SWWK CH2,