You can't plan for everything (and if you can, please call us - we'd like to know how). Imagine your weekly income is 750 and usually that works well. However, it's not uncommon to suddenly have a large unexpected expense, say - 700 for purchasing tickets on sale, or fixing an appliance.
There's nothing wrong with reducing your balance by 700 in the example above. Your balance might be negative for a few weeks until you recover from this unexpected expense, while you tighten your belt and make compromises because you're short on cash.
Payment plans are designed to reduce the psychological pressure of living with a negative balance; let's say 700 is used to purchase concerts tickets on sale, and the concert is 12 weeks from now. Why not spread the 700 to 12 weeks, reducing available spending amount by 58 until the show?