Yep, part of Banking Regs, you can tell them anything you want, but they don’t just type “Coke and hookers” into the system and file it away so, you know… use some common sense.
It’s a tiny part of larger system that at least tries to protect honest people. The Finance Ind which Banks are part of is one of the most regulated industries in the world. It’s not even a single regulatory body, there are loads, and a lot of the regs forbid them from disclosing what regs they’re acting within, or why!
They’re looking for lots of things, but a few examples.
Someone who may be vulnerable goes into the bank and asks for £1000 in cash to buy iTunes vouchers, is likely a victim of fraud.
Someone who deposits / withdraws large amounts of cash might be involved in tax evasion,
Someone who deposits large amounts of cash might be involved in crime etc.
Withdrawing 500 pound coins from a personal account would at least merit a question or two. From a business account, I doubt it would so much.