And then one day a barista at San Francisco’s Four Barrel Coffee explained it to me. He said carbon dioxide gets trapped in the beans during the roasting process, which gets released when the beans first come in contact with water. You don’t want carbon dioxide trapped in your coffee, because it contributes a sour taste. So you wait about 30 seconds until most of the carbon dioxide is released into the air, rather than into your liquid.
So much “common practice” in various fields is accepted without question. I think experts are often intentionally ambiguous, as if they are protecting the secrets of a magic trick. And yet, it’s so important for newcomers to learn the why, not just the what.