Photo editors on the iPad are intimate little buggers: you're touching and stroking pictures and seeing them react as if they were physical objects. Problem is, a lot of them end up skimping on features and feel bare boned. Luminance tries to do better.
Photo editors are pretty useful: boosting pics to bring out colors, adjusting settings to bring out quality, adding filters to bring out hipsters, etc. Snapseed for iPad is a great photo editor that's incredibly easy to use.