Why does NASA buy so much helium?

I'm curious to understand why NASA, the premier space agency in the world, invests so heavily in purchasing helium. Given its unique properties and applications in a range of industries, including aerospace, what specific needs or requirements does NASA have that necessitate such a large quantity of helium? Is it used primarily for fueling rockets, as a cooling agent, or for some other crucial purpose in their missions? Additionally, what challenges does NASA face in acquiring and storing helium, given its scarcity and the complexity of the supply chain?