"Save Slot" mode doesn't seem to be useful for anything, you can just create and go with custom and achieve the same effect.
The options that I see most likely to make Save Slot useful are the below:
1: If it automatically went as soon as you joined the lobby.
2: more people in the same lobby, so that you can make a save for someone else (or for yourself, with discussion about balance etc) without having it take your save slot, with unlimited time and 15 slots in the save.
P.S. Ranked. Patch notes say double the points, but game appears to give 1.5*, one or the other should be fixed :P