Here's why:

* Cellular Respiration: This is the process that breaks down glucose (sugar) in the presence of oxygen to produce energy in the form of ATP (adenosine triphosphate). ATP is the primary energy source for all cellular activities.

* Mitochondria: The process of cellular respiration occurs primarily in the mitochondria, the "powerhouses" of the cell.

* Energy Production: Without oxygen, cells cannot efficiently break down glucose and produce ATP. This leads to a lack of energy, which can severely impact cell function and survival.

In short: Oxygen is essential for animal cells to produce the energy they need to perform all their vital functions.