Another side benefit I'm not sure has been mentioned is the ability to quickly find past transactions based on category, and see when you last paid something. I moved into an apartment much closer to work about 5 months ago, but my single family home didn't sell for a while, so I was paying two sets of utility bills. I noticed via YNAB this week that the gas company had forgotten to bill me for the apartment for three months, so I was able to call them and quickly get it sorted out.
If I hadn't been paying for two properties I would have noticed much sooner, also.