Yeah it may run while floating, or not. If it does, it's just luck. Remember input pins have very high impedance, so if there's nothing to pull it to ground, then it'll run, but EMI or static or the phase of the moon XD could change that from one second to the next. Likewise, there's nothing to pull it to Vcc. So the resistor is to ensure the pin is at (or very, very near to) Vcc and stays there unless the button is pushed. Random un-commanded resets are definitely a thing to be avoided!