Rarity creates value. This is just a truism about human nature. If there is an item that only a few of us can have, it has more value. Collecting "hard to get" passport stamps hold a particularly high value for some travelers.
For those of us who are avid travelers, getting a passport stamp is the proof we were in a rare situation or achieved an extremely difficult objective. There are some passport stamps that are of more value and interest to others.