- I add a new paragraph (or new line) or some page
- Someone else also adds an new paragraph (or line) at the same place of the same page while I'm editing
This happens especially very often on talk pages or in project namespace, when people react to comments or vote.
In nearly all of those cases you want your paragraph to be added below of the one of the other persons.
Two-col-conflict will show you a conflict between both versions:
The current solution for this issue is to select on version, go to edit mode, copy the text of the other paragraph into it, click save twice.
This is considered non-intuitive ("why do I have to decide for one version, if I want both") and needs several extra clicks, what increases the risk of running into another edit-conflict when saving on a heavily edited site like votes or heated discussions.
Therefore it was proposed to have an option to use both version above each other.