Are you saying big favorite as in moneyline?
Yea, say if you like kershaw -300 but don't like laying that much, but want to par it with something else but it's the last game of the night and no other games are on the board...
Take Kershaw -300/open
so that when lines come up, you can close your parlay with whatever else. Either that day or later on...Not sure what the rules are on how long you can keep the parlay opened for...
Another reason would be if you think you will get a better line on a game and want to wait...
Hope this helps