Its all down to the big calendar conspiracy. All of this big calendar conglomerates want to mess with the days of the year to make you have to buy new calendars. Before 1921 there were no leap years and you could happily use the same calendar for a decade.
You're right, we should just make every year 365.2422 days long instead. So at 05:48:46.08 on New Years day, we put the clock back to midnight and leave it that way for the rest of the year. Then we do it again every year forever and ever.
Seems a lot less annoying than adding a day to the calendar every few years amirite?
Actually they serve a purpose.
It's been found recently (literally in this decade) that our Galaxy is spinning downwards, and it's been since the beginning of time.
Turns out that the gravitatory force of said spin affects our solar system too.
So, our Sun is spinning downwards too, and thus, our very own planet is spinning downwards.
This movement, of course affects our Rotatory movement. This create small differences between each complete orbit around the sun. Thus, each year our Rotary movement gets kind of misadjusted each year.
Turns out (and I'm not making this up) that every single 4 years we kinda reset back to our original place within the Orbit around the Solar System.
Thus, Leap years helps us adjust to that Rotary effect misadjustments.