[bravo22]

I am going to assume that you are OK with fractional reserve banking so I won't talk about why that is necessary as a function in an economy.

If I understand you correctly, you're basically saying why use a bank versus something else? My answer would be because for vast majority of retail bankers it doesn't matter. Same reason many would use Gmail vs self-host. They want to put their money somewhere and ensure that it will be there when they need it. As a society we have a framework around that and we provide deposit insurance and we call the orgs who take your deposit, give you an IOU (in the form of an account balance) then lend a fraction of it to others a bank.

For significantly larger sums no I don't think it makes sense to use a bank and most large corps have a treasury that specifically deals with that.