1. Qurbani is a practical expression of gratitude and monotheistic belief

During the era of ignorance, people used to make sacrifices for their false deities and would even invoke their names at the time of slaughter. Obliterating this act of polytheism of the disbelievers, Allah said:

وَ لِکُلِّ اُمَّۃٍ جَعَلۡنَا مَنۡسَکًا لِّیَذۡکُرُوا اسۡمَ اللہِ عَلٰی مَا رَزَقَہُمۡ مِّنۡۢ بَہِیۡمَۃِ الۡاَنۡعَامِ ؕ فَاِلٰـہُکُمۡ اِلٰہٌ  وَّاحِدٌ فَلَہٗۤ اَسۡلِمُوۡا ؕ

And for every nation, we have appointed a sacrifice; that they should mention the Name of Allah over the mute animals which He has provided them. So (remember), your God is One God, submit therefore only in His Majestic Court.^[1]

This means that Allah is your Provider. It is only He who created these animals, placed these powerful creatures under your control, and given you permission to slaughter them and derive energy from their meat. You have only one God, so only invoke His name when sacrificing animals, be thankful to Him and submit to Him! 

O those who love Allah’s Messenger! The favours Allah has bestowed us regarding animals, such as creating them, giving us power over them and making their meat halal for us; qurbani is a form of thanks for all of this. It is also an expression of monotheism. On the occasion of Eid al-Aḍḥā, Muslims around the world sacrifice animals in the name of Allah. They all declare Allah is one; the Bestower of blessings upon us is one; the Creator of all is one; we worship Him alone, without associating any partners with Him.

So qurbani is not a custom or festival, but an expression of gratitude for blessings and a practical demonstration of monotheistic belief.