The traditional form 備 showed a person with a quiver of arrows — equipped, ready. Simplified 备 keeps the footsteps 夂 above stores below: provisions packed before the journey. Preparation is arrows in the quiver before the hunt.
有备无患 'be prepared, no calamity' — from the Zuo Zhuan, 2500 years old and still the motto of every Chinese parent packing extra jackets. 备用 'backup' is why there's always a spare phone charger in the drawer.
Footsteps 夂 leaving with supplies stocked below — PREPARED for the road.
备胎 'spare tire' doubles as slang for a romantic backup option — the person kept 'in reserve'. Harsh, universal, very 备.