Well from a biological standpoint, altruism it helps your genes survive. If not by direct family (brother/sister/parents) than by your children, and providing them with an environment (friends, people, network) who is wanted to help them in life because of your altruism.
From a more ethical/humanist standpoint one could argue that since every human deserves equal rights. It is our duty to help others (less fortunate) and give them an equal chance of a good life. And apart from that, doing good, makes you feel good. In most cases when people are on their deathbed, they rather remember the change they had on the world and the people they have helped, instead of more trivial pursuits.
Hope this helps a bit.
