The reality is (I reckon) is that no one really knows why they do it and what for.
for years people thought it was a good method of weight control.
some think it makes one fitter, but for what?
at the risk of going all fadi and blabber fingers I just think "cardio" is an activity to become conditioned to the things we do throughout the week.
if you want to play a sport, or an exercise that requires more than your daily resources then a cardio program would be good to condition yourself to the demands that are placed upon you.
i don't believe cardio needs to be integrated into a workout, but like all things I do feel it needs to be added no and again.
having said that, I think cardio in a workout is the safest way to improve your capacity.