It's so easy to break things with them that they're probably best avoided, but they have been used to mostly solve several problems. Like, for example, implemen

It's so easy to break things with them that they're probably best avoided, but they have been used to mostly solve several problems. Like, for example, implementing preloading that can know which assets the client already has, along with a number of other caching-related tricks, and facilitating offline browsing of the site. From what I've observed through use (without any form of reverse engineering), it looks like Twitter uses them to reduce the bandwidth for individual page views after the first.