On my vocabulary cards I can write peculiarities of words. For instance, in English, pants is a plurale tantum, a word that appears only in the plural, i.e. there is no phrase such as “a pant”. One day a friend noted my pant leg (Note) sticking into my boot top and said something about my “pantalon”. So I asked him if that was normal, the singular, and he said yes (he taught Spanish and was a native speaker, too).
While those things may be acquired through reading, I do like the idea of noting them.
So someone keeping vocabulary cards for English would definitely want to note how one says pantalon b/c it’s very peculiar: pants leg or the leg of the pants.