[dagw]

You get a lower interest rate. Banks generally can't require you to have an account there to get a mortgage, however they can give 'discounts' to customers that also have their accounts at the bank. Close your account and you are no longer a customer and lose the discount.

[fest]

Banks frame it as "it's one of the factors used by our opaque black box algorithm to determine your rate". To find out the exact weight of every factor beforehand, I would have to submit N applications, processing of which is on the order of a week or so.

[dagw]

When I was 'shopping' for a loan (in Sweden) I got an explicit price list. As in "our base rate is X%, if both of you move your main account here we'll offer you Y% and if also you move your home and car insurance to the insurance company we own then we'll offer you Z%"