A bulb always seems to blow at the worst possible time – late night, weekend, or right when guests are coming. If you don’t have a spare, that corner or entire room stays dark until you remember to go and buy one. Sometimes that means days.
Keeping 2–3 spare bulbs of the types you use most – main ceiling, common lamps – means the fix takes 2 minutes instead of two days. You notice the failure, grab a spare, and move on.
It’s such a small “stock” item, but it stops your house from slowly going dim one failed bulb at a time.