Free app but need to purchase?

I was going to install an app using iPhone, and the app is free
But when I press the install button
There would be a sign writing: "Verification Required Before you can make purchases, you must tap Continue to verify you payment info ". Thn I tap the continue button, and it leads me to a banking thingy, like which bank am I going to purchase in.
But the app is FREE? Y should I pay an app which is free? XD
It happens to many app that I was going to install, some install successfully, some didn't:/ is it becuz I accidentally press something wrong at the settings or wat? O.o

You won't be charged if its free, thats just verification of your ID so make sure you have it setup then that won't come up again.