To draw in new customers, many banks offer enticing deals and sign up bonuses to all new customers. Of course they'll be some conditions that will qualify you for said sign up bonuses but if you intend to meet all conditions then as long as you are certain you'll earn higher interest rates at the new bank, why not move your account over, make some money [the bonus sign up], make a little more money thanks to the higher interest rates and move to another "better" bank the next year . . .
There are banks here with bonuses for new accounts but not the interest rate which is almost the same in all banks. In one savings bank, they have a premium in points for new accounts but only in dollar savings account and not in peso accounts (take note, our currency is pesos). And since I am an employee of a bank, I also invite my circle to open an account with us to earn some freebies and giveaways.
I don't like switching banks, so I wouldnt switch for a simple sign up bonus. Thats a gimmick. And I see no point being stuck with a bank I don't like over a few temporary bucks. A better interest rate would entice me a bit more, but the difference probably wouldnt be enough for me to switch.