Oxygen is important for a number of reasons.
- Oxygen is necessary for combustion to take place. Without it, there would be no fires to cook food (Among other applications).
- It is required for aerobic processes, otherwise all the processes would be anaerobic. The loss of oxygen absence can be gauged by the fact that aerobic respiration of glucose yield 38 molecules of energy (As ATP), whereas anaerobic respiration will yield only 2 ATPs.
- Oxygen is the best electron acceptor there is. All other alternatives yield much less energy.
- Oxygen is required for survival of aerobic life forms (all human beings and animals). It is also required for survival of aquatic life forms, including fishes and other zooplaktons.
- It is required for complete decay of organic matter. Without it, our wastewater and solid waste will not decay completely and will yield foul gases (among other issues).
Hope this helps.