I'm surprised at how many people think the answer is "no benefit" or political expediency.
The hope is that insurers will compete with each other and find ways to reduce costs. Obamacare has regulations designed to prevent the most egregious cost-saving measures (denying claims, rescission, etc), so insurers will have to find ways to reduce costs other than simply not paying. Because many insurers are small, they can try experiments that would be prohibitive in a larger insurer. We'll get a lot of data about what works and what doesn't, which will filter up to larger insurers (like Medicare!).
One example of a way that an insurer could save money is pay-for-performance, that pays health care providers based on the health improvements of their patients instead of volume of services. Aetna is trying this.
That's the case for a market. (There's definitely a case against it too.)
One thing missing from this discussion is the fact that the health care exchanges are small relative to the rest of the market: around 10 million enrollees on the exchanges, compared to ~150 million in employer sponsored plans. This is because Medicaid expansion covered more people, and fewer employers dropped their employee-sponsored health insurance, compared to predictions. So the exchanges aren't really a big player.