Okay, well maybe you don't but most people benefit from having one. The difference between tracking your expenses and deliberately planning what your money needs to do for you.
There's a thing called the envelope system, or zero-based budgeting, which basically means helps you assign the moneybags you have now, to the expenses you know you'll have. "What does this money need to do until I get paid again?" It's a whole thing, and it works really well.
I don't know how to link to communities but search for /c/ynab (or if you like, also /r/ynab over on that other site).
Agreed. I wish it was cheaper. Although now with YNAB Together my partner and I can share an account and my sister also agreed to have her budget. (I can technically peek as account manager but I've promised I won't and we're close enough that it's not an issue.) But now it's $30/each!