If you look on Right-Click Select (the feature request site for Blender), you'll notice that one of the frequently requested features is the ability to save or otherwise export just part of your active blend file to a separate file.
Export to Blend is a simple add-on that creates a new blend file based on your selection of objects, collections, or nodes. Exports can be marked as assets, which makes it easy to build out an asset library while creating a scene. Exports can also be backlinked, meaning that the original object in your scene will be replaced with a linked version from the new file. It works from Blender's File / Export menu and from the Outliner's context menu.
Creating a new file with just one asset is very helpful for managing data in larger projects. Let's say you're working on a scene, and you've added a prop to it, but then you realize that the prop is going to be necessary in multiple scenes. If you use Export to Blend, you can simply push the prop to its own file and quickly link it to your other scenes in just a few clicks.
Development of the Export to Blend add-on is publicly available on GitHub. Please use that repository for bug reports, feature requests, and pull requests.
Now, go have some fun!