Apologies if I'm wrong, but I was under the impression that with these textures, solar system objects would appear as detailed as any object outside of the solar system.
You're wrong. It would be impossible to have them appear as detailed as procedural objects, it would require terabytes of data, and anyway that data simply doesn't exist.
If this isn't the case, what purpose do the high res textures serve as I can't see much difference.
If you applied them correctly, Mercury, Earth, Moon, and Mars should be noticeably more detailed. The Moon is probably easiest to notice. If you can't see any difference, then you either did not install the textures correctly or you did not make a good comparison between the two, because the difference between the default and the addons in those cases is formidable. You can see here what the Moon should look like with the addon pack, and here what Mercury should look like.