Youtube Offline Viewing allows you to download youtube videos and watch it later on your iPhone and iPad without Internet Connection. If you want to watch a Youtube Movie, Music Videos, Podcasts, Interviews you can download them right from the video window and view it without internet connection later.

Also Watch, Share Facebook Videos on WhatsApp in iPhone: https://www.youtube.com/watch?v=ocuSrgfccQE

Not all the videos are enabled for download by video publishers. So you will be able to download only videos allowed by uploader's. Still its a necessary feature for youtube app users.