If you're editing a question, and someone else saves an edit before you do you can only save your edit if it is "more thorough" than the previous one. This is the message you get:
Now imagine that you are editing a long post and you have corrected a dozen spelling mistakes, or other such minor things. If the edit that was saved while you were doing that was significant enough, you won't be able to save your edit even though none of the things you corrected were addressed by the other edit.
So, if you still want to make all those corrections you have to reopen the question and then manually make all the edits again. But wait! What if you simply don't remember all the edits that you were making? What if it took you fifteen minutes to find all those problems? You now have to start over, reading through the post again to find the issues. This might be a big waste of time.
(Of course you could copy and paste the entire post from the box where you were editing it, and then open up another version of the post and individually copy and paste all the changes made by the saved edit into your version. But that could also take time, and I somehow imagine something going wrong when copying and pasting thousands of characters.)
It would be very simple to redo the edit if there was a way to see the changes you made even without being able to save them. The same way you can access the revision history after an edit is saved, and see what was changed, you should be able to see a summary of your changes even if they can't be saved. This way you would be able to quickly see all the changes you made, and redo them in the blink of an eye.
(Yes, this did happen to me.)
Related: The editing race - the slackers always win , Improvements to a suggested edit should be saved even if the suggested edit was approved , Improvements to a suggested edit cannot be submitted when the suggestion is approved during editing