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.
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.