Well, way back in the history of PHP it was necessary because the slashes added by the browser where not automatically removed. Then there was a setting called magic_quotes and you could turn it off to keep the old functionality or have it on and PHP would remove the slashes. Now there is no setting and PHP always removes them. If you’re using an older version of PHP you may still have this setting and stripslashes is a way to make sure they’re removed. Why ACF is still doing it, not really sure. If you are using an up to date version of PHP on the server then it shouldn’t be needed. If you want more information start here http://php.net/manual/en/security.magicquotes.php
