A proper way to use a button and an LED - General Electronics - Arduino Forum

Could somebody please explain the reason the following circuit works "fine" (LED is on if the button is pressed) only if the Pin 12 is in the OUTPUT-mode? I need to be able to digitalRead() the button state (pressed/not pressed) and the LED should be on every time the button is pressed without using two pins (one input and one output). What is the proper way to achieve that?

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