Pull up/down resistors

[blueskull]

Why do we need pull ups or pull downs for input pins? For STATIC operation, such as boot mode selection, can not we simply connect the pins directly to Vio or Gnd?

For what I can think of is powering up a pin several ns before Vcc MAY cause latch up? But I always don't care about it, and it works all the time.

[Neilm]

Quite often, the pins can have multiple functions and it could cause damage to the output stage if the pin was accidentally driven. You could end up with a dead short through the IC. The resistors will limit the current in this instance.

[baoblackcoal]

Sometimes need enough current.

[TimFox]

Pull-up resistors: 
Original TTL and some other logic families were unhappy if the input voltage exceeded Vcc (for example, during turn-on) unless there was a current limit.  A TTL output worked well, but a resistor was recommended for permanently pulled-up pins.
Pull-down resistors:  With TTL, the resistor must be relatively low due to the high input current at VIL.
It is often considered good design to use a small pull-down resistor to ground, even when not needed, in order to allow in-circuit testing of the device with a controlled voltage or pulse to that pin.