It means that the last time a post or page has been edited by anyone, it shows on the post/page, as well as the original creation time. It is useful to keep track of things and see if anything has been altered. From my (Strangely Perfect) viewpoint, I make changes to the original articles just as I make changes to recordings. That is, it’s quite hard to be really satisfied with anything as after time has passed, I can always think of new bits to add.
Other reasons might be to add or remove some defamatory or complimentary remark in light of changing circumstances.
Or even amend or add a fact, or a spelling mistake …