With newer DSLRs having the ability to be remote controlled from computers and even pump out video preview of the image using Live View, it was inevitable that someone wrote an app for Android to do all of that. Unlike similar apps on iOS that actually require a computer in between the camera and device, DSLR Controller lets you remote control a compatible DSLR directly via USB.
This app is definitely out of the ordinary as far as Android apps go, so compatibility is complicated for both the Android device and the camera. That's why the devloper has a stripped down remote shutter app available for free, which also works as a compatibility tester. You can also consult the compatibility chart, which also lists which cameras are compatible. There's a lot of maybes with whether or not this will work, so you're just going to have to try. For my, my rooted gaalxy Tab 7.0 Plus works fine, and has done so since I first tried this app months ago, while my Galaxy S II which isn't rooted does not work.
Assuming you actually get to the stage where you have the two connected, the app is quite feature-packed. Imagine all the settings you have available on your DSLR. Now imagine those settings beings assigned to buttons and whatnot on your Android device. That's essentially what this app does, and it give you full control over your camera. You can even transfer images over once shot and use the glorious screen on your device to view them.
As for what you'd use this for, the possibilities are endless. Be it macro photography or simply controlling a tripod-mounted DSLR during a photo session, I'm sure this can come in handy for a lot of photographers of different skill levels.
The app is classified by the developer as a Beta right now, currently at version 0.92 (so not far from release), but has been available that way for months. The idea is to raise the $7.99 price once version 1.0 is released, so this is a good time to grab it before that happens, as upgrades are of course free.