Is it? Only pure water will actually freeze at 0c. Rain, puddles, lakes, etc aren't all that pure... And we're talking about ambient air temps here. The air can be below freezing and it can still rain. And you can get snow/hail above freezing...

Knowing the freezing point is just one factor. Knowing it's generally around 30F is pretty much always close enough (not that remembering 32 is actually very difficult)

Edit: also water only freezes at 0c if it's at sea level... I really don't think 0°=freezing is the huge advantage that celcius stans think it is.