This is to provide an explicit option to not use the system proxy setting for IMAP connections, as described in T7430. This was originally intended to be implemented for IMAP as well as SMTP and POP3, but it got overlooked. https://bugs.kde.org/show_bug.cgi?id=407685 refers.
D21463 implements the proxy option in the IMAP core library. This is the GUI for the setting, which appears as an additional option in the "Advanced" tab of the account settings dialogue.