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.
It refers to takeoff from an intersection rather than using the full runway length. The part of the runway behind you is now useless. It is part of the saying because the TODA (takeoff distance available) is now reduced and this needs to be taken into account when doing performance calculations. It is e.g. discussed in this thread on pprune.org.
One could ...