Under which circumstances does a user answer two different answers on the same post?

When I think about it, I believe I've already edited some old answers of mine to add more solutions (that I've learned along the way since the time of writing the original answer) so I really can't see why this button is needed.

Edit:
I would also assume that it's forbidden to answer more than one answer on the same post because for example, the way to gain a badge for let's say "Linux", requires you to answer a certain number of answers which are marked with that tag, so it seems to me like a way to fool the system to get your badge quicker.