Lightroom is probably one of the reasons many photographers use a Mac. While Adobe does have Lightroom for the PC, many photographers prefer the Mac's workflow.
Expect Apple to keep its hold on many shutterbugs now that Lightroom Mobile is available on the iPad. Of course, it does image editing, with the star feature here being RAW file editing. You can easily sync to Lightroom on your desktop as well as the Creative Cloud. You can also access your camera roll on the iPad to make edits and sync them to your desktop.
All in the Creative Cloud
Don't misunderstand, though. Lightroom isn't meant to be a standalone app. You need a copy of Lightroom installed as well as a Creative Cloud subscription. If your version of Lightroom isn't a Cloud-based one, you'll need to subscribe to the Creative Cloud, which starts from US$50.
For photographers who want to access their Lightroom collection while on the job, Lightroom Mobile makes sense as it saves them from lugging their computers around just for Lightroom.
You can download it free from the App Store.