Every single personal money management system starts with the same advice, that you have to make a budget and stick to it.
It makes sense, but for a lot of people budgets are just paper exercises because they have absolutely no idea how much they need to spend on their lives.
To manage your money you have to know what money comes in and what money goes out but so many budgets get blown by the first little crisis.
The argument always is "I absolutely needed it so I had to buy it" and if that absolute need is a new clothes for a child that had an unexpected growth spurt or to pay a vet bill then yes, you do need to spend the money.
But what about if your best friend is getting married in the Bahamas and wants you to be her maid of honor or if your son has a chance to go on an amazing, but costly, school trip?
Once again you might look at what you thought was a great budget and realize that it doesn't work and just abandon it.