Originally Posted by
BAAZ
As far as I know, all it does is it stops the checkin process before you confirm that you want to check in. Until you click on confirm check in, you aren't checked in. Just switching off your phone or closing the app has largely the same effect. The option isn't available to you if you are actually checked in.
If you check-in and confirm your seat choice but don't print/download your boarding pass, you still have the option to Cancel check-in and exit. The same applies if you haven't printed/downloaded your boarding pass and you go back in to see what seats there are.
In both cases you
are checked-in (and the website will say so), but you still have the option to Cancel check-in and exit because the check-in process isn't fully complete because you don't yet have your boarding pass.
For the same reason, the main MMB page will say something like Check-in in progress even though you are checked-in.