Showdown Value

[Acemanhattan]

This is a concept I greatly misunderstand, it is very fundamental, yet I just dont grasp it. At any point in a hand, we either have the best hand or we dont. If we have the best hand, wether it is bottom pair or top why do we consider our showdown value? Shouldnt we be getting as much value for the best hand as possible? And if we dont have the best hand why do we want to get to showdown with it at all? Someone please give me a solid description of what showdown value is, and what it means to our game.