Weak pull Up 12F629

What is the difference when you activate the weak pull ups?
Because if you make a pin an input, isn't it tri-stated?

So can't you apply 5V directly to your pin without activating the weak pull ups or am i being wrong.

I think i am, because i can't believe they make something that doesn't have any function.

Thanks at advance

weak pull ups are resistors (10k) pin to 5v+ though the resistor
if you just add 5+volts to the pin and took the pin low then you would be taking 5v+ to ground basically a short circuit
maybe I some were in the ball park