When the user initiates a commit, Magit calls git commit without the
--message argument, so Git has to get the message from the user. To
do so, it creates a file such as .git/COMMIT_EDITMSG and then opens
that file in the editor specified by $EDITOR (or $GIT_EDITOR).
Magit arranges for that editor to be the Emacsclient. Once the user finishes the editing session, the Emacsclient exits and Git creates the commit, using the file’s content as the commit message.