There is no obligation in Halacha to give found money to tzedaka, according to the letter of the law you may keep it for yourself. However, giving the money to charity is a win-win proposition because you get credit for giving money that belongs to you to tzedaka and the person who lost the money also receives credit for enabling your donation.
It is impossible for a human to know exactly how much credit he receives from Hashem for a mitzva, but we know that the reward is based on the challenge that was overcome. Presumable it is easier to give away money you found (easy come, easy go) than money you earned. Still the money is legally yours and you are giving your money to tzedaka.