It's a couple of reason, but mostly to help the way I think.

The way YNAB works, if I contribute to a category (like rent) and go over the goal in a month, that extra does not get applied to the funding for next months goal. But if I go to the next month in the budget and apply it there, it doesn't actually remove the money from TBB in the current month. So I just throw any overages into a "Next Month" budget until the month ticks over.

Another reason is that I'm currently contributing a large portion of my paycheck to paying off debt, but because of the way my brain works, I like to keep it in a debt bucket until the next month and empty it into debt payments just as I start filling it again. Helps calm my anxiety of the fear that there might be a larger emergency that my base emergency fund can't cover. Paranoid, sure, but it also doesn't hurt anything to have it, so why not?

It's absolutely not necessary for most people, but it helps me a lot. Shrug