>Isnt the air you exhale CO2?

No. It has a little less oxygen and a little more CO2 than what you breathe in, but still much more than enough oxygen to be more useful than nothing.

• Regular air (what you breathe in) is 21% oxygen and about 0.04% CO2.
• What you breathe out is still 16% oxygen, and about 4% CO2.

So what you breathe out is still only 4% CO2! And it still contains about 3/4 of the oxygen you breathed in, too. Yes using fresh air would be better, but there's no good way to force in fresh air with no equipment. Delivering 16% oxygen with 4% CO2 by mouth is the best available easy option and better than nothing.