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Why do UK banks require monthly “pay in” into current account?
From the banks point of view the point of a current account like this is to get you as a regular customer. They want to be your "main bank", the bank you interact with the most, the bank you turn to first when you need financial products and services, the bank whose advertising you see every time you log into online banking or walk into a branch. The bank knows that if they just offer the unprofitablly high interest rate or other perks with no strings attatched that people will open the account and dump a bunch of savings in it but won't actually move their financial life over, their old bank will still be their main bank. So they attatch strings like a required minimum deposit, a minimum number of direct debits and similar. These have minimal effect on people actually using the account as their main current account while being a pain for people trying to game the system. Of course as you point out it is still possible to game the system but they don't need to make gaming the system impossible, they just need to make it inconvianiant enough that most people won't bother.