VMware Content Libraries are a 6.x "new" functionality (I use quotations because 6.x has been out for a while) that I've really grown to enjoy. Content Libraries allow you to store objects such as ISOs, templates, etc. in a central repository that can be shared with other vCenter environments. One of the biggest use-cases is to clear up all those random "ISOs" folders on various datastores as engineers, who are searching for an ISO, look in just the wrong folder first and create their own. 

When Content Libraries were first released, there were a few issues that made it difficult to use. The main issue was that in order to use an ISO in the library, I had to download it first, and then connect it to through the web client. For that matter, I may as well just continue to use the locally stored ISOs on my system anyway. You could still deploy templates from the library, which helped, but ISOs were what I really was looking forward to. 

vCenter 6.5 (and 6.7) fixed this and allowed you to connect ISO images directly from a Content Library. When I stood up our 6.5 cluster internally, I started setting this up and created a repository on a FreeNAS device with the share exposed via NFS so that I could use some cheaper storage to hold this content. Growing excited, I tried to create my first VM with an ISO image stored in the library. And it didn't work. Turns out, in order to connect an ISO from a Content Library, the backend storage must either be a VMFS datastore directly connected to the host so that the VM can "see" it.