[blawson]

Maybe at certain sums much larger than individual depositors concern themselves with, you can't just "hold the cash".

Like banks might say there is no way we want your $10 billion in cash to look after. Either invest it yourself or pay us to invest it for you.

[josefx]

You really don't want $10 billion in a bank. Laws mostly guarantee a few hundred thousand per account holder if the bank itself has financial issues. You would have to spread that amount over quite a few banks if you wanted it secured.