I don't understand how the mapping works. An address has 8 parts and produces 16 words, so each part consists of 2 words. If we take the example 2a02, that gets encoded to "how atop", but I don't see how that text helps me that "how atop" means 2a02? Am I suppose to memorize both? How does that help?

You are not supposed worry about the mapping. You trust the website to help decode it. You just remember the sentence. It's a little like what3words for coordinates.

The rationale being you are more likely to remember grammatical cogent sentence, than a random string of alphanumeric characters. Although I will agree that the generated sentences don't seem easy to remember. So I doubt it's utility.