Efficiency.

The most honourable and wise Maestro that I am blessed to study under was recently injured. Being the kind of man he is, he could not abandon his students.

So, on his sick-bed, brought down by a sprained ankle, he had a revelation.

Pinning a foot is a training tool we haven't used until recently. The idea is- one person stays still, and the other comes in and tries to stab. Historical terms could be Patient and Agent... Hindering the back foot forces one person to be the Patient.

CappoFerro says that there are tempos to strike in (yes, I'm mixing terms, so what). Generally, as he steps, as he goes to strike, after he strikes and... stuff. Nailing your foot to the floor shows this quite clearly. If your distance is spot on, the agent will get a sword to the face every time- why?

Because, just as it is hard to pat your head and rub your tummy at the same time, it is hard to step forward and parry at the same time.*

*This may not be true. If you have set the blade so your strong lay by his weak, you may be able to turn the blade offline in the natural progression of a step. Thus parrying, stepping and killing in the one movement.