ELI5 Nitrogen - if it’s 80% of our atmosphere, why don’t we talk about it much?

We talk so much about oxygen and carbon, but 80% of the atmosphere is nitrogen. How does it fit in? Why does it not get talked about? Why does 1% of the world’s energy go into speeding up the nitrogen cycle? Why do divers know a lot about nitrogen? Why is liquid nitrogen so useful?
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