Here's the biggest example of an app to close an iOS functionality gap: Instagram.
Because iOS still doesn't do sharing properly! Completely pointless on Android. I take a photo, share it to any number of vastly superior photo editing apps, maybe through several to get all the edits I want, then share it to any social media network I prefer, instead of creating a whole, new, redundant network for the purpose! Use Andmade Share, and I can simultaneously share it to fb, g+, twitter, gmail, gdrive, drop box, flickr, tumblr etc all in one movement!