For most people it's because they live paycheck-to-paycheck, at least after things like 401(K) contributions.
Of course if you are lucky enough to have $30K+ coming in per month and your necessary monthly bills are closer to $5K then budgeting and tracking spending out of the checking account is wildly unnecessary.
But if you have say $5K-$6K coming in each month and $4K of that goes to necessary monthly bills then checking in for anything more expensive than a tank of gas ($60 or so) starts to become necessary.
Why would having separate accounts eliminate the need to communicate for larger purchases for couples in the latter situation. Sounds like that would lead to even bigger issues.
Separate account transactions can't accidentally cause a critical bill payment to fail from the bills account (joint or individually managed). They de-risk the effect of small purchases, which it's fine for one person to keep to themselves, on the bill payments which affect all partners.
Purchases might cause insufficient funds for topping up the bills account later, but that's on a longer time cycle, and someone will know about it. So it doesn't eliminate the need to communicate about large purchases (or the sum total of many small ones), but it does eliminate the need to "check in" before individual purchase decisions, and eliminates showing each other the fine details of all your personal transactions when each of you sees the statements of your joint account(s).
E.g. some people are happier buying a bagel every day for lunch, or maybe an ice cream, or trying something else, when their partner is not posted an electronic trail that shows each bagel or ice cream purchase every day, exactly what time of day the transaction occurred, and price accurate enough to infer number of fillings.
It doesn't in terms of agreement on purchases. But it makes the actual logistics of balancing the checkbook easier. You definitely know that you are or are not overdrawing.
(fwiw I use joint accounts, but I can see the benefit of separate accounts if both people are working or otherwise busy and there isn't time each night to balance the checkbook.)
Of course if you are lucky enough to have $30K+ coming in per month and your necessary monthly bills are closer to $5K then budgeting and tracking spending out of the checking account is wildly unnecessary.
But if you have say $5K-$6K coming in each month and $4K of that goes to necessary monthly bills then checking in for anything more expensive than a tank of gas ($60 or so) starts to become necessary.