Smart Objects mean you can make non-destructive edits to an image, and keep edits in separate Photoshop file
Save an image with its layers intact as a PSD, and then open a different image.
Go to Open As Smart Object and find the saved layered file. You’ll notice a small symbol on its layer, indicating this is a Smart Object.
Double-click on its layer to re-edit the PSD, and hit save to apply any changes and close it down.
This keeps all the layers out of the way, freeing up space in the Layers palette of your main composition.