I still really don`t see the advantage of this, it just serves to upset and annoy everyone. Surely we should be able to change any of our old posts, or delete if we wish! Sorry, but I still get annoyed with this.
Imagine it's like you're writing for a magazine. You can change stuff before you send it (and, of course, unlike a magazine, for a little while afterwards), but once "printed" that's it, it's printed, forever amen.
The main reason for preventing edits & deletions of old posts is to try to preserve context. Some people are prone to flouncing off, deleting their posts behind them. There's rarely a sensible reason behind it, and all it does is leave a whole bunch of threads and posts which lack context, and - often - are rendered useless by the missing info.
Ultimately, boards like this are a resource for the members - I can go back to threads that are 1, 2 or more years old looking at techniques and ideas - which I can't if someone's flounced & taken the posts away...