Around my house, we're pretty good at budgeting. We set out and decide how much we're going to spend on various categories, and usually approximately "hit the mark". The days we get thrown off our budgeting game, though, are the times things don't go as planned.
One month, a couple years ago, we had a death in the family that was going to require unexpected travel, and cancelling other trip plans we had made for that same weekend. Due to the hurried nature of the trip, we didn't get our cash for our envelopes out of the bank, saying instead we'd just use ATM cards this month. At the end of the month, we were shocked at how much we spent. Yes, there was a trip involved, which means eating out more, and getting back from a trip can always be hectic and normality can take a few days to return, but this was a whole-month problem. Our schedule had been disrupted, our typically-well-followed plans were askewed, and we compensated by spending more.
On a typical work day, I'm home by 4 or 4:30- a full hour or hour-and-a-half before my husband, with plenty of time to get dinner ready. On the few days that I work late and don't get home before him, we often look around the kitchen, see that dinner isn't ready, and go out to eat instead. Last night was one of those days. I was at the office until 6, and my husband had already gotten home. If it were a typical month, he'd let the dog out, feed the dog, and come pick me up for a quick dinner out. Last night was different. There was a whole chicken roasting in the crockpot, and he heated up some green beans, then brought dinner to me at the office, where we had a strangely-romantic dinner in the breakroom.
This is what No-Spend Month is doing for us- making us "think different". When our plans get thrown off, the default answer is to spend money to compensate. With a little creativity, we don't have to.