One of the many, many, many tasks you can do in The Legend Of Zelda: Breath Of The Wild is to collect dragon parts from the three mythical beasts flying around Hyrule so you can make armor, special elixirs, get into places, etc. It is a time-consuming process that's a pain to achieve with all three beasts. So when you've beaten the game and have nothing else to do, it helps pass the time.
One player discovered a nice little trick you can do if you collect enough dragon horns: you can make a fireworks show. GirKart64 took all their horns, put them on top of a mountain with a bomb, detonated the bomb and created a nice little show. There's no trick to this beyond wasting the time to collect this many horns, but it's nice and pretty. Just another example of all the cool little things you can pull off with the game if you try hard enough with their open world universe and cool mechanics. Good job, player!