We evolved in the Savannah.
Rain means the watering holes are filling up, which is obviously good cause we need water, but it also attracts prey animals.
This, of course, was summarized most eloquently at the zenith of human evoloution: the 1982 hit single by Toto clearly stating, "I bless the rains down in Africa."
I'm still missing something here. For it to be useful, I'd imagine that it would need to inform decisions, and do so where existing senses would fail.
At least in my environment, if I can smell rain, I could also just as easily use my eyes to see the cumulonimbus clouds and say "rain, due east".
In the savanna are there scenarios where the only awareness of rain would be smelling it? Can you derive directionality at 5 parts per trillion? Does it matter?