Post Library is where you can create drafts of posts (text, images, links etc).
If you want to quickly save a link or an image that you want to share/schedule later, you can do that in Post Library. You can also use Post Library to post a link, a text, an image to your social media channels.
How does it help?
Post Library helps in two ways:
1. you can save anything (and everything) in your post library and post/schedule it any time (even repeatedly)
2. you can save posts as drafts and come back later to edit them before sharing / scheduling them to your social media channels.