Crop, mask, or erase parts of content and photos
Just a few examples: I would definitely want to apply, for instance, a paper texture behind (or into) a frame. If the frame is hand-drawn I may want to create a mask that follows the contours of the frame. Or if the frame and texture are of different proportions I may want to crop the texture so it fits. Or I may not want to scale the texture down, so I would create a mask to reveal a portion of the texture at 100% of size.
To mask objects (e.g. pull a travel stamp of your passport while removing the background). This could all live in the in app camera (if there is one).