Since the two phenomena that define a year (one rotation of the earth around the sun) and 365 days (365 rotations of the earth around its axis) don’t match up perfectly in time, we need a way to account for the difference.
There’s also a lesser known factor called the leap second which takes care of slight differences in the atomic clock.
Imagine what the world would be like eventually without leap day. In a hundred years noon would be the middle of the night and we’d be getting out of work as the sun rises. Without the leap second though, changes like that would take several millenia.
Photo: Camera Eye Photography, “Week #6 “Leap” [6of52]” February 9, 2012 via Flickr, Creative Commons Attribution