Money that WE (the public) invest with banks and building societies is then invested by THEM to earn ever more interest.
For example banks lend money to other banks overnight, or for short periods, and earn more interest from that than they pay to you and I.
Also during a recession more people default on their mortgages or loans, which costs the banks money.
The banks "write off" millions of pounds a year in loans that are never paid back, either through the person going bankrupt, or cannot afford the mortgage, or through fraud and so on.
They need us to keep investing with them to cover for the money they have lost through loans not paid back.
In fact some banks, like Northern Rock (and a few in the USA) have lent so much money to people who have then defaulted on the loan (like mortgages) that it can cause the bank to "go under", or at least write off millions or even billions in bad debts.
They need money coming in to cover for that.