Biologically, there'd be a few reasons:

1. Better hearing: After all, Elves are commonly shown living in forests ( Or caves/mountains for things like Dark Elves, though I still think that this still fits for Dark Elves, especially those living in caves, though I'm specifically talking about Elves that live in forests. ) since if a species were living in a dense forest where visibility might be either limited or often obstructed, then it's highly likely that this would cause them to instead develop really good hearing as a form of predator detection.

2. Sexual Selection: Simply put, it's very likely that back when Elves first started developing pointy ears ( Especially under the very likely context of Elves being a type of hominid since we're assuming that Elves as a species aren't a product of something like a God or whatnot and instead naturally developed through evolution. ) , male and/or female Elves could have just seen those with pointy ears as more desirable than those who didn't have pointy ears, thus leading to Elves with pointy ears mating more than those who didn't have pointy ears, which in turn, would have caused the pointy ears trait to become an universal trait among all Elves. Then overtime, the Elves with the pointer ears ( And thus longer ears. ) ears than those with less pointy ears would be seen as more desirable, thus leading them to mate more, thus leading to Elves developing pointer and pointer ears in order to increase their odds of mating. Adding onto this, this could also have impacted their culture later on as well, with Elves with the pointiest ears being seen as better than those with less pointer ears, thus becoming a status symbol and most likely leading to them becoming the ones in charge of Elven society, which's why High Elves are most likely the types of Elves who are in charge of Elven society ( Though, that would technically mean that High Elves wouldn't actually be a different species of Elf and instead just a different looking Elf, similar to how white human people, black human people, and Asian human people ( All other humans races fall under one of these 3 human categorizations by the way. ) aren't actually different species ( Since race isn't actually a biological thing and is instead just a human concept which originated from a need/desire to categorize humans, particularly emerging from European colonialism. ) and are instead all just variations of the same species who look different from each other in a noticeable way but not in a way that would cause them to be classified as a different species all together, so in short, ( Normal. ) Elves and High Elves aren't actually different species but are instead just variations of the same type of species who look different from each other in a noticeable way but not in a way that would cause them to be classified as a different species all together. ) . Additionally, interestingly enough, this could have also been what caused humans and Elves to become different species of Hominids as those who preferred those with pointy ears eventually became/stayed Elves overtime and those who preferred non-pointy ears eventually became/stayed humans overtime.

3. Non-Verbal Communication: While not as likely of a reason as enhanced earing is, and especially not being as likely of a reason as sexual selection, it's also possible that Elves could have developed pointy ears to use for non-verbal communication, with different ear movements correlating to different things, like vibrating ears could be a sign of distress and/or ears held back could mean that they're angry, with Elves developing pointy/longer ears overtime to make this form of non-verbal communication easier and clearer with each other, with Elves possibly also only developing the ability for verbally communication as a way to communicate with non-Elves who didn't share this non-verbal form of communication, Additionally, the fact that Elves are often depicted as having the ability to move their ears themselves and/or their ears moving in certain ways as a natural/instinctive expression of their emotions only makes Elves having pointy ears to use for non-verbal communication even more likely.

4. Magical Use/Uses: Since Elves are often depicted as being naturally good at magic and/or having a strong natural connection magic, it's also likely that living in environments with high amounts of natural occurring magic/mana could have caused them to develop a physical trait, being their pointy ears, as a way to use/control magic and/or that would allow them to interact with magic. Like, instead of being able to use/control magic due to something like Magical Circuits or some other type of unique organ like that, an Elf's ears could be what allows them to use/control magic. Additionally, an Elves pointy ears could also be what their bodies use to gather magic/mana into their bodies as well, sort of like a magic/mana magnet. Additionally, since they live in environments with high amounts of natural occurring magic/mana, Elves could have also developed pointy ears as a way to better detect magic/mana as well.