Branded power supplies are made by manufacturers who put their name on them for a reason. The fact that you don't see why there would be a difference is gravely disturbing in the computing world. Lower quality power supplies made by no-name companies, or cheap knockoffs are made with lower quality components, and due to the nature of how a power supply works, if those components aren't of a certain quality, you introduce things like noise, ripple and component failure which can not only cause a power supply failure, but in the case of cheaper power supplies, actually take out other components in a system. A friend of mine cheaped out on the power supply for a high end computer, and the PSU fried on him one day, it took out his motherboard, his CPU AND his video card and hard drive. His "savings" turned into a liability because he didn't spend the extra $30-50 on a high end power supply. It's a type of insurance that really does matter.
A good gauge of power supply credibility is its weight. Cheap units will weigh less, because they don't use as many heatsinks and quality components, which tend to weigh more. It's not the only indicator, brand names fall into play as well, but generally between reviews online, the weight of a unit and the name brand can be used to gauge a good unit. Here's a couple of links that go into the whole deal on these devices:
Everything You Need to Know About Power Supplies | Hardware Secrets
How to choose an ATX Power Supply - PC Power Supply - How to Select Computer Power Supplies-Best Computer Online Store Houston Buy Discount Prices Texas-Directron.com