"Did you know that once you've collected ten of these Secret Memos, the numbers contained in them will combine to form a special code? I'll be in touch again to explain how to use the code, so be sure to make a note of it once you have it!"
Having collected a handful of them already, I figured they were mostly just fan service. The Internet seems to have magical puzzle-solving powers, so someone out there will crack this, I'm sure. Miyamoto also reminded us to "check out Mission and Bingo Battle -- they're a lot of fun too!" He really wants us to play Bingo Battle, doesn't he?