When cutting an external thread with a die, the die must be started and held exactly square (perpendicular) to the axis of the rod and then turned evenly around it.

A die stock is a purpose-made holder that clamps the round die securely and centres it, and its two long handles let the operator apply balanced turning force from both sides.

This balanced, centred grip keeps the die aligned with the rod as it advances, so the thread it cuts is straight, true, and of uniform depth all round.

Gripping the die directly with pliers holds it at only one point, so the die tilts and wanders, cutting a crooked, drunken thread that will not fit its mate.

The real benefit of the die stock is therefore the stability and alignment it gives, not a change in cutting speed (option 2) or a reduction in weight (option 1).

Nor is extra torque the point (option 3): the handles do give leverage, but a lopsided grip that lets the die tilt would ruin the thread no matter how much torque were applied.

Hence, the answer is Die stock provides stability and alignment.