Explain the origin of the leap year. Why is it necessary?

(So I just realised I accidentally commented I really dont know how to use this.)

Explanation:

So I don't know the Origins but this is why it exists.

It takes 365.25 days for the earth to orbit around the sun. That extra quarter isn't a full day, it has been decided that every four years, to make up for that. 25 an extra day has been added to the calendar. (.25x4=1[days])

The origin of the leap year are based on the calendar of julian related with gregorian calendar, which are basically introduced by the Julius caesar in the 45 BC. So, the leap year is the year which contain one additional day in very fourth year.

It is necessary because in the even number of the days the earth does not revolving around sun. So, the situation of severe accident can arise if the days of earth's rotation is in a whole number for earth rotates around sun.