Mouth is an obvious answer for me. Sometimes when I catch a cold, my nose gets congested to the point that I can't breathe AT ALL, like 0% air can get through

luckily we have a mouth, which produces NO MUCUS and that we can use to breathe whenever we want.
Also, if we only had nose how would we be able to play any sports or do exercise? We would be exhausted too easy and fast.