Well, what if you paid someone $150 by check (say a tax bill or you borrowed from a friend). They drag their feet cashing it (tax departments tend to do this). You look at your balance online or whatever and it says you have $300, so you totally can use your debit card to buy a PS3. Except the day after you buy your PS3 the person holding the check you gave them cashes it, and now you’re in the red. And you owe fees because you just caused a check to bounce.
Until a check is cashed, there’s no record of it outside of your own personal records. That’s different than a debit card, which takes the money out of your bank right away or a credit card which just piles up until you pay it.
If you never, ever use checks or if you use them so little that you can keep track of them in your head and subtract them from your balance until they’re cashed, then you probably don’t need a register.
Me, I just did my monthly stint in Quicken (I don’t use a paper register anymore, but Quicken is a register) and had to enter 6 checks that I’d written since 2/8/11, only three of which had been cashed already. And trust me, I pay everything online if I can or use credit cards. This was all for other stuff.
So yes I keep a register. Seems like you have to write a lot of checks if you’re a homeowner.