In Agrippa’s day the primary concern was defending oneself from a cut. In order to better facilitate this, one would hold the arm somewhat back and at an angle to cover more of the body.
Agrippa is primarily concerned with thrust. To make it more effective, he instructs his students to hold their arm straight. The diagram below shows how one’s reach is shortened by a bent arm.
The knee is shown again in case it wasn’t clear enough in the previous image.