When you're trying to explain things to people who don't understand math. When the measures you use are not valid. When you don't know how to interpret the tables and analysis. All of the above. Interpretation is something we can all do.
Interpretation is common-sensical.
Interpretation requires a solid understanding of the mathematics involved. Interpretation confuses people. Interpretation requires you to do some essay writing. Answer in 25 words or less. Answer in 25 words or less.