By using Stamp Visible you can merge layers together and edit the merged copy, while still having access to the original ones if you need them
When you’re working with multiple layers you can use the Shift+Ctrl/Cmd+Alt+E keyboard shortcut to activate the Stamp Visible command.
This creates a merged version of all your layers at the top of the stack – just like the Layer>Merge Visible command would do.
However, Stamp Visible has a big advantage over Merge Visible. Merge Visible flattens all layers in the Layers palette, rather than placing them on to a new one.