If somebody pulls on your arm and you are employing a 70/30 stance, you will eventually topple forwards unless you step.
By stepping you avoid bracing, and maintain central equilibrium.

It is very hard for an opponent to exert force on you or manipulate your body when you move as they move.

Sinking to one side allows movement to flow; being double-weighted is sluggish.
Anyone who has spent years of practice and still cannot neutralize,
and is always controlled by his opponent, has not apprehended the fault of double-weightedness.
To avoid this fault one must distinguish yin from yang.

(Wang Tsung-yueh)