It's supposed to be where you are going to a specified event/function and don't want to feel left out so much. You plan on the number of syns you are going to allow yourself (20, 30, 40, 50 etc) & stick to it - right it off and thenget right back on plan.
It is meant to be a one-off type thing - not a weekly or monthly splurge.
Personally, I'd rather save up some syns to help ofset such an event but the above is what SW allow.