I have thought exactly the same thing. In fact, here are the notes from
some research I did into this a while back:
The verse feels somewhat out-of-place if it's nothing more than a comment about what God equipped Adam & Eve with for the journey ahead. If, however, a biological change was being made here, that certainly warrants being mentioned.
Now, for one thing, it's important to use a good translation. The KJV says:
Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them.
Whereas ISR, overall a much more literal translation, says:
And YHWH Elohim made coats of skin for the man and his wife and dressed them.
Notice that in the ISR translation, you have skin (singular); KJV renders it plural because it is being assumed that animal skins are involved here, contrary to the actual Hebrew. Also, the word clothed/dressed is important. This is Strong's H3847, which says: "לבשׁ לבשׁ, la^bash la^be^sh, law-bash', law-bashe', A primitive root; properly wrap around, that is, (by implication) to put on a garment or clothe (oneself, or another), literally or figuratively: - (in) apparel, arm, array (self), clothe (self), come upon, put (on, upon), wear."
So the literal translation of this word is "wrap around." That's fitting, because we don't just have skin. We have layers of skin. The term epidermis literally means "over or upon the dermis." Wikipedia says, "The epidermis serves as a barrier to protect the body against microbial pathogens, oxidant stress (UV light) and chemical compounds and provides mechanical resistance. Most of that function is played by the stratum corneum." This manner of barrier (even if we're talking only about the stratum corneum) would not have been necessary within the hospitable confines of the Garden of Eden, but certainly would be necessary for the long-term survival of humans outside of it.