It's easy, only takes a minute and — if you make a drastic mistake in any part of your existing site, or worse yet, accidentally delete the whole thing (yes, sigh, it's possible) — it can save your bacon!
Using an archive, you can quickly recreate any part of your site. The trick is to archive frequently, because you can't restore what was not saved.
Click on "Archiving Sites" in the right-hand column keyword section, below, or see previous post titled "How do I Archive my Weebly Site?" (12/27/2011)