Because they have to know who you are in a fair bit of detail both to comply with the law and to know who they should let take money out of the account. Because

Because they have to know who you are in a fair bit of detail both to comply with the law and to know who they should let take money out of the account. Because they need you to agree to a bunch of contacts. Because they need to get you things like a bank card. Because they need to decide if they want to lend you money (most often in the form of a credit card). And so on and so forth.