Armpit sweat does not have a strong intrinsic odor. What you smell is the waste of the microorganisms which colonize your armpits. It is a similar mechanism responsible for foot odor and bad breath. All external bodily surfaces have particular microbes which prefer different areas. This also includes the intestinal, oral and nasal regions.
If you charted these microbes on our body, their colonies would read like a map of microbial nations. Different lipids, sugars, proteins on individuals promote different types of microbes and different metabolic pathways giving individuals different scents.
Our ability to perceive a wide range of microbial odors might have an evolutionary pressure given the importance of detecting microbes in spoiled food, etc. I hypothesize our ability to detect body odor is inherited from detecting microorganisms in general.
Surely as our olfactory sense has diminished over evolution, these microbial odors are being taken out of some ancient context.