In the case of the bundled Epson versions of ColorBurst, you can easily gang up multiple prints on one piece of paper. That's basically all the RIP does for general photographic use. Great use for $500, right? (You can also relinearize the provided profiles if you drop the cash on a UV-cut i1 Pro, but that looks less useful with every generation of Stylus Pro.) Most of the features are lost on you if you're making photographic prints for exhibition or sale.
Speaking more generally, a RIP may provide better print quality, interpolate more transparently, offer more exacting control over ink amounts and color mixing, kick butt at B&Ws, allow prints to be made by simply dropping files into a folder... Print quality's largely a wash today, though, with manufacturers providing considerably better driver output than they used to.
Most of the popular RIPs are available in functional trial versions, so if you're particularly interested in one, grab the demo and check it out once you have the printer. Because their primary market is in networked production environments and proofing, though, the price:feature ratio is decidedly skewed when it comes to an individual photographer making his own prints.
