If you start writing a commit message with -m, then realize you want more paragraphs, editing at the command line is slower, or that escaping quotes is trickier, you can use -e to take what you have so far as a starting point in the editor.
Or, I don’t know, do not use -m at all and it will open text editor for you?
The use case of scripting commits is the first that comes to mind where you don’t want that, but might want multi-line commits.
Interesting, though can’t say I’ve ever needed to use multiple -m flags.
I’ve generally used the shell’s built-in multiline string syntax for this. Lead the string with either a ' or ", and the shell will continue parsing new lines as a part of the string until it’s closed.
Someone mentioned this feature on a slack a few months ago and I’ve been using it ever since.
It’s really nice because I can also use amend and move around the message using my vi keybindings without ever needing to drop into the editor. But at the same time I can do the summary/details split in my message.