Android malware has previously used overlays to hijack taps and do other shady stuff.
Banking apps generally set a flag on their apps to prevent overlays and screenshots to prevent malware from reading the screen and tricking the user.
Depending on your phone, this can have two effects: either the overlay is disabled automatically or the banking app detects the overlay and blocks access (ie to the PIN keypad).
Not all banking apps do this, but the ones I've used do.
