In Photoshop, a Smart Object is a layer that has been transformed into a referenced external file. So any changes you make to the referenced file (in this case, the top-down galaxy map) update live in the main Photoshop document without having to reimport anything. Any additional filters, distortions, and effects applied to the Smart Object are non-destructive.

In contrast, if you have a normal layer and perform, for instance, a Gaussian Blur, you can't go back and edit the pre-blurred layer—the effect is permanent.

Smart Layers and Smart Filters were a tremendous advance in Photoshop workflow. I personally wouldn't go back to any version of PS that didn't have them. Not being familiar with Gimp, I don't know if it has the ability to reference an external file in this manner.

To address the actual map, though:
Troedel: I think the very saturated purple cast to the galaxy as a whole is fighting with your region colors. I suggest toning down that color and letting the regions take more prominence. Also, you have some visual interference between the Perseus Arm label and the title in your cartouche. If you can find a way to reduce the vertical height of the legend, that will keep your text from overlapping.