The Scout Law states that A Scout is Thrifty. The Scout Handbook goes on to say that Scouts work to pay their way and to help others. They save for the future. They protect and conserve natural resources. They are careful in their use of time, money, and property.
Paying your own way with money you have earned gives you independence and pride. Even if you only have a few dollars, you have enough to save a bit for the future and to share a bit with others -- although what you share doesn't have to be in cash. Volunteering your time and talent can be just as valuable as donating money.