co2 isn't as good for mig as a proper argon/co2/o mix, however it does work well enough for lots of applications.
Pure co2 does create a hotter weld, however that can be a problem if dealing with thin metal.

The main benefit of the proper mix, is it has a trace of oxygen (2% IIRC), which helps burn of impurities.
Ideally you should have all metal ground clean, and free of oil, but that's not always possible.
With pure co2, any impurities can cause excessive splatter/porous welds, whereas a proper mix is a bit more tolerant and minor impurities will burn out into the flux instead of affecting the weld.

And for the benefits of Birchy, I also have a bit paper that says I can weld. Having a bit paper that says you can weld 6mm bits of plate in designated ways, doesn't mean you can weld, as was demonstarted by some of the numpties who managed to pass the course the same time as me.