I started YNAB when I wanted to get more serious about saving for college for my son, and toward building a house on my property.
I could "just save x for each per month", but I work a lot of overtime, and some holidays (so get paid extra), and want to optimize my savings without sacrificing too much of my lifestyle now.
So, I have everything as a line item. I budget for known bills, things that are going to come up (car maintenance/repair, home maintenance/repair, household goods, etc) and really try to minimize any need for "crap, I need to pull money from savings to cover this".
However, if I have something bigger come up than I've budgeted for, I'll move money from other areas (on a road trip, I knew I'd use more gas, so I lowered how much I set aside for gift giving for the month). If I work a lot of OT, I'll add a little extra to the areas that seem low.
I realized recently that I hadn't accounted for a new cell phone within my "cell phone" budget line, so had to pull from others to make up for it. Now I've added a little extra each month to that line, so that it should cover a new phone when I have to do that again (hopefully not for many years).
Groceries is an area where I'm close each month, but I might go a little over on occasion. Unless I work a lot of OT, I just let that balance rob from the next month and make up for it later. It usually balances itself out fairly quickly, so I don't worry about covering for it from other budgeted categories. If I were to go over every month though, I'd know I'd need to take another look at either my self-imposed budget, or my spending, because one of those would be off.
I saved over $28k toward the "new house build" alone this year, so budgeting has definitely been working for me. If I'd just kept with "save x amount toward the house per month", I wouldn't be anywhere close to that. A few months ago, when I realized I might be able to save to a "round $50k" by the end of the year, I made that my goal, and really kept myself in check with hobby spending, wasting money grabbing fast food, etc. I rewarded myself for reaching that goal by adding the extra to my vacation fund, and booked a (very frugal) trip to Sweden.