The first reason is that it is easier for users than having to type in their PIN or a password every time they want to print something. Instead, they only have to remember their PIN when registering a new swipe card.
The second reason is that it is faster to log in with a swipe card than a PIN. It makes sense to make a process as quick as we can, it may only be a few seconds faster, but those seconds can quickly add up to minutes or even hours.
The last reason is that it is much easier to remember a swipe card than a PIN. People will often put the card in their pocket, and leave it there until needed, so as long as you have pockets of a reasonable size, you’ll never be at the printer wondering where your swipe card is.