This used to be a lot more common with Pocket PCs and feature phones. Opera Mini was very popular for devices that couldn't render a page on their own, or wanted to save on bandwidth. I guess there's still a market for this on low-spec devices.
They're going for the niche of people who want to do serious web browsing on mobile, don't have 100$ to buy a half-decent mobile device, but are still willing to pay for a service?
Used to use some of these browsers on my ipod touch to bypass middle school web filtering.