This is an often misunderstood or completely overlooked panel, but it has some useful functions once you take a closer look. The most important thing to know about this panel is that it simply provides different ways to view/access photos that have gone through the import process already. These are not duplicates of your photos, but just alternative ways Lightroom can reference the source files and display them for us.
This collection is a quick way to view all of the photographs (and videos) that you’ve ever imported into your catalog. If you ever need to run a filter on your entire library, just click on this collection, switch to Grid view, and use the power of the Library Filter to display just the photos you want to see.
All Synced Photographs
If you are a Creative Cloud subscriber, and you’ve synced some photos with Lightroom mobile, then all of those synced photos are gathered in this collection. You can even sync new photos with Lightroom mobile just by dragging and dropping them onto this collection (though I find it preferable to add photos to specific synced collections to make it easier to manage).
You can also remove photos from being synced by clicking All Synced Photographs, then selecting the photo(s) you no longer want to have synced, and pressing Delete. You’ll see a popup dialog warning you that the selected photo(s) will be removed from all synced collections and Lightroom mobile, but not be deleted from the desktop catalog. I occasionally use this to remove photos from being synced that are not part of any synced collections (such as photos you took with the Lightroom mobile camera app while not in a specific collection).
If you need a quick way to temporarily gather up groups of photos for specific tasks, then the Quick Collection can be very useful because it has a super power. By default, the Quick Collection is set as the Target Collection, which means that you can select a photo anywhere in your catalog, press the B key, and that photo will be added to the Quick Collection. Let’s say you want to export copies of photos that are stored across multiple different folders, so while in Grid view, you click through each folder and when you find a desired photo, you select it and press B. At the end of the process all the selected photos will be visible in the Quick Collection for easy export.
Right-click the Quick Collection to access a contextual menu with options to clear the collection or save it as a regular collection. Speaking of regular collections, you can right-click any regular collection and set that collection as the new Target Collection. After you’ve set another collection as the target, you can always right-click the Quick Collection again and reset that as the target.
If you’re in the process of importing a new batch of photos, then the default behavior is to switch to this collection so you can see them as they are added to the catalog. Why switch to this view? Well, you might have the Import screen set up to save the imported photos across multiple sub-folders, so by using this special collection you can see them all in one place regardless of where the photos are stored. The contents of this collection changes after each import.
There are also a number of temporary collections that only appear when certain things happen. Such as when you export as a catalog or search for missing photos (Library > Find all missing photos). These functions generate temporary collections for gathering the related photos. If you no longer need to see these collections, you can right-click and choose Remove This Temporary Collection to clear it from the Catalog panel.