What is the purpose of an MBA for IB, if you already work in IB and have a bachelors in economics/finance/accounting/etc.??

I used to work as an IB analyst, and one of the things I have always wondered is why larger investment banks have a 2 year analyst program, and force everyone to go do an MBA for 2 years before being promoted to associate. I've never taken an MBA before, but I did do a bachelors in finance, from doing some research online it seems like the concepts taught in an MBA is very similar if not the same as the ones taught in a bachelors degree.

Which makes sense because MBAs accept people from different backgrounds and everyone needs to be up to speed with the same concepts. But if you already have a bachelors degree in Finance, Accounting or Economics and already work in a large IB, then isn't it a waste of time to relearn the same things again, when you could be gaining work experience and getting paid?? I found alot of the theories we learned in school are just theories, they don't really apply in real work.

I understand if you don't have a business background or are trying to break into IB or Consulting, etc., then an MBA makes sense.

Please enlighten me.