When an answer is marked correct the topic is marked answered and show a green icon. So I think marking any answer is better than not marking any. But the marked answer is displayed just under the original question. So marking the right post as correct not only lets users searching the forum with a similar question know there is an answer but makes it easier to find.
I think it's not really about points or credit to the person that gave the correct answer. It's about making the forum more useful to people searching for answers.