Media Library is where save all your imported media files. However, it functions more than a place that displays all the imported media files, Sample Colors and Sample Videos. It also allows you to manage your media files in folders.
By default, a folder is built beforehand for all the media files you're going to import. You can build more folder or delete a folder by clicking the "Add Folder" or "Delete Folder" icon.
It can be used as transition, intro or credit for a video. You know that a transition can only be used between 2 videos, not for the beginning or the end of a video. If you want to add an intro or credit for a video, sample colors are helpful.
There is 1 sample video for "Countdown". It can be merged to your own videos.
Note: Media files can't be edited in Media Library. To edit them, you must drag and drop them to Timeline area first.