Why is "runway behind you" useless? I was recently answering this question and for some reason it made me think of this fairly famous aviation saying


  The three most useless things to a pilot are the altitude above you, runway behind you and fuel on the ground


Now, I get altitude above you - you may as well use some of it lest you meet a mountain. I also get fuel on the ground - if in any doubt fuel up before you go.

But I'm not sure I understand why "runway behind you" is useless in this context

Could someone explain?
The runway ahead of you is useful to accelerate and - if needed - to decelerate.

If you enter (or touch down) the runway in the middle, you will not be able to benefit from the part of the runway behind your entering point. It is the same as if that part would not exist. Therefore it is useless.