Good question, and no one really knows, but there is an hypothesis that we had a significant period locked near water (maybe an island) and may have been a catalyst for part of our evolution, Probably in the north of Africa near or in the Mediteranien (sp?) during a very wet period. the nose structure would have enabled us to swim more efficiently.  It is called the aquatic ape theory. Some of it's explanations seem to be jumping at straws, but I think it does bring up a good point, that we only have theories built on theories as to why our noses got the way they are, why we are furless (also explained as aquatic in this hypothesis), what exactly drove our brain evolution, and why we sweat differently then most other mammals (we sweat profusely through sebaceous glands which are only used in the underarms and palms in apes. (the idea put forth for that one is that we were on an island in a salt sea and had to supplement with salt water for a long time, sweating out the toxins was the only way we survived, but our phenotypes have abadoned some of the extreme sweatiness that would have allowed us to survive on mainly salt water.) ) So that's my long explanation as to why our noses might be that way. Unfortunately hypothesis is about as close as you will get to an answer on that one.

For what it's worth-

I once lived near a tidal pool that warmed up in the afternoon due to the large black basalt rocks which kept the pool tranquil and warm. There I discovered I could actually fall asleep while floating on my back in the water- so long as my arms were 'over my head' behind me. In this posture, on the in-breath, my face rose above the water entirely. On the out-breath, my nose remained above the water; since I could be safe in both phases, it was possible to drift off to sleep.

Our noses allow us to breathe in the rain. If your nose was upside down it would not work well on a rainy day. On rooftops, where the air conditioning system has an intake for fresh air, the duct comes up and over to point downward for the same reason.

So that we don't fall at the first hurdle?

From watching my children breastfeeding, I noticed that their protruding noses allowed them to breathe easily even when their faces were pressed tightly against the breast.

The relationship between, benefit, risk, taste, smell and lifespan. Perhaps investigate events that may be or may not be mutually exclusive.