Why do we need morality? What purpose does the concept serve?
Why do we need morality?
Well morality at it's core is the way in which we try to answer questions of what should be. Although other fields of study concern themselves with what is, ethical inquiry has to do with what should be.
This has far reaching consequences for humanity. The prevailing morality of a society has great effect on it's citizens. Not just do we need morality in this world, we need a good one to boot.
What purpose does the concept serve?
A very important one it guides our actions. It helps us discern right from wrong. Our chosen moral compass is the basis on which we judge our actions.
The answer that I would like to give is:
[...] [T]he biological function of moral judgment is social coordination[.]
Arguments for and against (and other, related, arguments) can be found here.
My rough take on this: Societies that have a suitable moral code (aimed at coordination, but that also includes a rule that some individuals punish, e.g., ignore, other individuals that do not behave according to the moral code), are likely to do better in evolutionary terms. That doesn't imply that all individuals will or "should" comply with this code at all times (say for the sake of society). That doesn't matter, as for most individuals in most circumstances, it is (individually) rational for them to obey that code.
For further reading, I might also suggest:
Some not-very-structured thoughts.
We have a very deep-seated sense that there are things which people can be blamed for, and things they can be praised for, that there are right actions and wrong actions, and that some states of affairs are better than others. Philosophy, concerned as ever with clarifying our notions, establishing whether what we think is true is in fact true and trying to provide an analysis of of what makes it so, should certainly aim to figure out what's happening when we're making all these claims, even if its conclusion is that they are meaningless - that's part of the study of ethics
It seems to me that the assertion that there are things we ought to do is very tied up with the idea of things mattering; what happens matters, and that generates obligations to act in certain ways. So if you think anything matters, or have any sort of conception of the good, that naturally leads you to try to analyse what sort of relationship actions can have with the good.
I don't think we as humans can escape from having a sense that there are right and wrong actions - that is, we cannot not have moral beliefs. We'd have those even without a heading called 'morality' to put them under, and we'd have them even if we didn't make them public and discuss them all the time.
I would say morality can be understood to serve as a standard against which the rest of human endeavors are judged. If we "need" anything, we need morality; what would it even mean to "need" anything, if it's just as well not to have it as to have it? What would it mean for logic to say that some statement is true rather than false, if we don't place any particular value in truth over falsehood? When people argue that morality isn't needed, I think they're referring to a very narrow understanding of morality that has to do with some school or other's own pet philosophies.
Broadly speaking, you might take morality as the source of the basic distinction between opposites which makes it possible for variety to exist in the first place. The is/ought distinction seems like a related, if not the same, notion: without morality, things are; with morality, things ought to be.