Help me understand laptop battery life (details in question)

It has been explained to me that you should always let the battery run completely down before charging it. I understand this in concept. Where I get confused is using my laptop with the power cord. So if I am at a desk and I can plug in the computer, does this somehow impact the battery life? An example, I plug the computer in and start if up. Now I have to use in but have to disconnect it from the power cord. Should I then run on battery until the battery is out before I plug it back in even if I am going to be using it again back at the desk for a longer period?

Is there ever a good time to just use it with the power cord plugged in or should I always be using the battery?