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Austro-Afrikana, to show you what I mean, I borrowed this image from Google Images (I'll take it down if the owner recognizes herself and wants it down)--just send me a PM:
Notice that blonde extension hair is used, but you can see the model's own hair lying side by side with the extension hair in the braids so that both colors show up. In each braid, her own hair as well as the blonde extension hair are exposed to wind and friction--like if she were to let her hair hang or wear a hat that rubs against the braids.
In the braid she has tucked behind her ear, you can see that her hair doesn't reach the end of the braid but it stops somewhere behind the ear or maybe just before. Just like her hair is exposed for most of the length of the braid, so too will that end be exposed, meaning it's vulnerable to the elements just like any hair that isn't buried in a bun or some other protective style.
So what braids afford her is the freedom not to constantly comb her hair and cause it undue trauma. So it's a low mani style not a protective one.
Cornrows are protective for the most part whether with or without extension because particularly if the cornrows are long and winding, most of the ends of the hair are buried inside the cornrow so only the tail ends hang like extension hair and so aren't protected. Weaves are protective styling because besides there being cornrows, you also have the hair forming this blanket that keeps hair protected from the elements.