"and as showers upon the grass": Just as showers descend upon the grass and beautify and preen it, so, words of Torah beautify and preen their disciples. And thus is it written (Proverbs 1:9): "For they (words of Torah) are a chaplet of grace to your head," and (Ibid. 4:9) "It (Torah) will set a chaplet of grace upon your head."
Variantly: R. Yehudah was wont to say: One should acquire words of Torah as (general) principles and "expend" (i.e., "implement" them as particulars, it being written "My taking (Torah) ya'arof as rain," "ya'arof" connoting acquisition. An analogy: One does not say to his neighbor "P'rot" ("break") this sela (a coin) for me, but "arof" (acquire) this sela for me. Similarly, one acquires words of Torah as (general) principles and implements them in small drops, like dew, and not in large drops like rain.