And I would ask what valid reason is there to not allow someone to change a post indefinitely?
Regardless though, let's say someone writes a "how to" post, and a year later finds out something new, and people like to refer to the post, and so this person updates the post with new information, so eveyrone does not have to be notified of a new post and have to try and find it. I don't know. I am sure there are plenty of reasons why someone may change their post at a later date.
Also, if someone quotes a post, and someone changes their post after it is quoted, everyone can see that it was changed.
"Most voters know nothing about how markets work—or even that they work..." Sheldon Richman