Having multipack birthday cards will save you the burden of having to go to the store every time to buy one card at a time for coworkers, relatives, or friends on their birthday. It will also exempt you from the stress of trying to think of what to get for this or that person.
When you already have a birthday card pack at home, you don't need to worry about who has their birthday next. Once you find out about a birthday, you are at liberty to pop one card from your drawers, make it out to the birthday boy or girl and deliver it either by mail or in person. Now you can expend your time and effort choosing other gifts rather than waste it by the card shelves in the supermarket.