The transaction from the brokerage said that the funds were deducted immediately from my account, which was easily verifiable. It also stated that it would take 3 days for the funds to show up in my bank account — equally easy to verify as that's how long it took.
So — in those 3 days, who had my money (presumably earning interest on it)? While my transaction wasn't huge, multiply it by a few hundred thousand transactions a day and it's likely a pretty huge sum.
Bonus ask — when I can compose an email and have it traverse the globe in a matter of seconds, why is it banking of any form still takes an eternity? I hold an account at BoA and they state some transactions can take up to 8 days. This seems insanely long. In my mind it's simply because banks use this "float time" to make massive wads of cash for doing absolutely nothing.