I’ll try to go for a true ELI5 here. Life needs oxygen to burn food. Burning food gives living organisms a lot of energy, which they can use to move around, or grow fast, or break up some more food etc. it’s not strictly necessary though, life started out without oxygen, and there are many living things (bacteria and others) today that make do without it. There are other ways to get energy, doing it with oxygen is just much more effective. Organisms like us can’t live without oxygen because we came to rely on it.