maybe you have never lived through a period of sustained financial stress. Yes, people in that situation understand that it takes a few days for a check to clear. One of those checks is your paycheck, which you need to clear to cover your other costs. The other is the check you wrote to the grocery store so you could eat, or your rent check. Which clears first? Did you time it right? What if your paycheck was delayed by a day?

Money in vs money out is very hard to keep track of when you don't have a buffer and you are always balancing empty.

I want to make an analogy to a seemingly simple buy known hard computer science problem, like resolving bugs caused by race conditions. Stuff which is simple if you can assume a single processing thread can become a nightmare when this assumption does not hold.