For example, let’s say I budget 50EUR per month of fun money, i.e. 1.67EUR per day. (I was a very poor student a long time ago.) Suppose it’s the start of the month and I have no backlog. This concert comes at 35EUR including transportation. 35/1.67=21, so I’d have to wait until to the 21st of the month to buy it. Since I really want the concert, I take 35EUR out of the fun money and put it into the committed-but-pending-expenses money.
Let’s say I also want a candy bar for 1.67EUR. Candy isn’t healthy, so it comes out of the fun budget, too. Fun money is at -35EUR right now, i.e. at 21 days of backlog. So I’ll have to wait until the 22nd to buy the candy bar.
Do I want the concert or the candy? There’s no right or wrong answer, but the budget forces me to make a choice. Or I could increase the fun budget, if I can find a way to spend less on something else.
Fortunately, the spreadsheet handles all the complicated maths. (On an unrelated note, it was my learning project for Excel, and I really enjoyed tinkering until I had effortless bookkeeping. Except syncing with the bank statements - there was no way to automate that back then.)