For those who have separate finances with their spouse and contribute a certain amount (say, 50/50) towards bills: Why?!

My wife and I have been married for 15+ years and we have never had separate finances. All of our money goes into a central pot and we take care of groceries, bills, and miscellaneous stuff. If my wife wants to spend some money on something, I don’t count pennies or tally it away. I guess I don’t understand why you all keep it separate. You’re a family unit once you’re married, right?