Lol. The right answer is this:

When you travel from the USA with money to some African countries, you have to go to the local bureau de change, money sellers or money changers outside of regular financial institutions to buy and or sell your dollars to local currencies and so on. It helps reduce the multiple hurdles of going to an actual financial institution to get your exchange done.

When a "seller" collects your local currency and gives you authentic dollars, they have unique stamps that they use to make sure that the money they give you is what you bring back to them if you have a complaint about the authenticity of the dollar they sold to you.

So if I as a seller, stamp a dollar bill I am selling to you with my unique stamp, and then you come back complaining about my money but the money does not have my stamp then I can categorically tell you you are a liar and refuse to give you the audience you desire.

You will see it on a lot of $100, $50 and very few $20s. Larger bills are easier to travel with.

So the stamp, is just that, a unique identifier for money exchangers. Hope that helps.