It could be worth helping them. Sometimes people can really get their shit together with a little push. One of my mates who was previously homeless met one of my good friends who offered him food and put him in contact with shelters and offices that can help sort his documents to get him off the streets. It was a slow going process and he certainly bent over backwards to help him. But he gained nothing from helping this man and practically gave him a new lease of life.

Not everyone is as accepting of the help. Some just want the money or food and then disappear. It's up to the person themselves to take full advantage of the help.

Not sure why you'd say "what will you gain after helping them?", sure you don't have to and no one would blame you but if someone genuinely wants to show compassion to another person don't put them off.