system
Given the following post, respond as if you're commenting on Reddit:
human
[Thor] Why can Heimdall see and hear everything, unique among his people? Who else can?

What also is his role in Asgardian society? Is he the only one who can see and hear so much? Is his ability a result of genetics or technology? If he's the only one with this ability, why isn't he more closely guarded as a vital asset to Asgard, but instead acts as a guard to the Bifrost? Does he have the freewill to choose his role?

Is anyone else in Asgard training to replace him one day?
gpt
Heimdall is naturally magic, like all Asgardians. He can see and hear pretty much anything except things that are shielded from him. Mind you, he also needs to know what to look for.). He is the only one amongst the Asgardians who can sense that much without using magic or the Odinforce.

There is no one being trained to replace him because the Asgardians were aware that Ragnarok would occur some day. That and no one was born with the abilities TO replace him. His sword is, however, the equal to Thor's hammer in every way, and he is capable of fighting the Thunder God to a standstill if he tries, making him one of the most powerful beings in all of Asgard. This explains why he's also out there as a guard, very few beings that would march on the Golden Realm through the Bifrost could sneak up on him or force their way past him.

He chooses to stand guard on the bridge, but it is also his duty to do so. Odin ordered him to and he cannot refuse such a command. However, he can and has taken the rare break to listen to music, eat with the others, and spend time with his sister (the lovely lady Sif!). He even ruled Asgard for a short time when Odin and Thor were unavailable!

Also: Heimdall is forever fit as long as he eats of the Golden Apples that all Asgardians require to survive.