While this thread's content isn't directly related to D&D, I'm basically just overanalyzing a couple characters I found on Twitter, it's so easily translated that I think it's fair to say that it's at the very least analogous.

I have a friend who has a tiefling character and she retweeted some art that she had made of her (and a few other characters, including two other tieflings). I mentioned this fact and she playfully said "I'd say I have the mightiest of horns of all of them" which for some reason reminded me of how Charles Darwin observed how birds from different regions have different beaks depending on what type of challenges they might face finding food. This got me to thinking about what the evolutionary purpose of a tiefling's horns would be, because each tiefling in the original image was depicted with relatively different horns.

The original image, drawn by @ClarissaWalace on Twitter (my friend's character is on the bottom left):

The biggest difference here being @Mystery_Celeste's character, who's horns are more short but seem a lot tougher and sort of ribbed maybe, while @UwUkeksi and @ShioriTamayomi have smoother horns, with Keksi's having cracks on them, so they've been damaged somehow. Perhaps they were used against hard objects or structures, something Celeste's character would be more equipped to do with the more robust horns, albeit smaller ones. Also notice the angle of the horns, it's a little harder to see with Keksi's drawing, but Shiori's go out, then curve up like a hook, while Keksi's curve a little less. Celeste's don't do much curving at all, they're moreso just spikes. Also, a last little observation, Keksi seems to have elf-like ears, which perhaps suggests that she's a half-tiefling/half-elf, though it could easily just be a design choice.

Essentially, my ponderance is: What practical purpose does the horns of a tiefling have, and how would certain factors affect how they manifest, and how could this fact be used in a campaign? For example, perhaps a tiefling could use their horns to make an unarmed attack that deals some amount of piercing damage, like a tabaxi's claws?

I'm pretty sure Lore Wise, that teiflings don't get their horns from an evolutionary origin. It is genetic from fiendish ancestors. The different variations of horns, could be a result of different planes. For example: Abbysal, Chthonic, and Infernal, with infernal being the default one. Being how they are all Anime girls, they could all be descendants of Chthonic Tieflings.  The reasons they would have different horns is the same reason we would have different faces, It's a genetically inherited trait.

What exactly is a Chthonic Tiefling?

While Infernal Devils are lawful evil, Demons are Chaotic. Chthonic tieflings are in between Infernal and abbysal with nuetral evil fiendish ancestors. It is described in the players handbook that many of them possess unearthly beauty like a succubus.

You're underselling them.

Tieflings who have the chthonic legacy feel not only the tug of Carceri but also the greed of Gehenna and the gloom of Hades. Some of these tieflings look cadaverous. Others possess the unearthly beauty of a succubus, or they have physical features in common, with a night hag, a yugoloth, or some other Neutral Evil fiendish ancestor.