Plugs can be finicky, especially when you need to connect more than one. It's easier to just plop a gadget into a dock than connect a power cord, audio out, video out, USB, and whatever else you'd otherwise need to connect. Using a dock also reduces clutter on your desk, and, from a purely aesthetic perspective, just looks more neat. I've got a dock for my smartphone and one for my portable media player but not for the most-used gadget of all--my MacBook.
When I work, I like to plug my little 13-inch laptop into a big monitor for more screen real estate. But that means I've got a ridiculous number of cords running from my laptop into various accessories on my desk: a power cord, video out to the monitor, audio out to my stereo, and USB for keyboard, mouse, external hard drive, and printer. It's unseemly. It's cluttered. It feels like I've cobbled a computer out of spare parts I found at a garage sale.
Henge's dock is super simple, but has a lot of great uses. Instead of plugging all that nonsense in when I want a desktop experience (and then unplugging it when I go portable), I could just slip the laptop into the sleek dock. It's got all the ports on the left-hand side of the MacBook: power, ethernet, FireWire, USB, mini-DisplayPort, and audio. Slip it out (carefully!) and the laptop is back. It's also a nice solution to have next to a TV--you could install Boxee or some other media software and have an HTPC without having to pay for any more hardware.
The Henge dock is only compatible with Apple MacBooks; right now, they've got one for the 13-inch unibody MacBook and MacBook Pro, and the 15- and 17-inch unibody MacBooks Pro will be coming soon, as well as the plastic models (both black and white). It's kind of a specific gadget, but it provides a really easy way to get a desktop experience on the cheap. Buying a monitor and this Henge dock is about a thousand dollars cheaper than buying an iMac, if you've already got a MacBook.
The 13-inch MacBook Pro Henge dock is available now for $65, while the unibody 13-inch MacBook (last-gen) is available now for $60.