It makes you completely in control of your finances, and it focuses your mind on your spending habits.
An example: I can get out of the habit of organised food shopping. I can end up in a cycle of 'popping to the shops' because I haven't planned my shopping. That's annoying. However, because I have YNAB, I split my shopping categories by the shop I spend money in. My local Sainsbury's is around 20p per item more expensive than the Sainsbury's 5 miles away/online. Of course, any Sainsbury's is more expensive than Lidl/Aldi. Over the months, I've become more mindful about planning and I can see, through YNAB, not only the reduction in my overall grocery spend, but also the shift in my spending from more expensive shops to cheaper shops. That gives me more money to allocate to other areas of my budget.
With YNAB, I can easily see the impact of decisions on my overall budget - I'm accountable to it. If I try to spend more than I have, it points out that my spending is unfunded and asks me to correct it (perhaps the Government should have YNAB?)
Say you're saving for Christmas. You have allocated £100 per month to Christmas savings. You are invited out to lunch. Probably £20. When you look at your budget, you see that the only category that you can adjust is Christmas. It makes you think about it - Is going out for lunch worth having less to spend at Christmas?
By having a mindful budget, I can create categories for those annual costs that pop up unexpected. For example, Amazon Prime. £95 once a year is a fair whack if you're on a tight budget. £8 per month put into an 'Amazon Prime' category means that come Match, the money is sitting there, waiting to be spent on that bill. Same with MOT costs, YNAB cost, etc., and suddenly there are no nasty months.
Overall, the aim is to get to the point where you are spending last month's money this month, or even the money you earned before that, so that you aren't ever caught short.