Expressing "now" with "le"

下雨了 doesn't mean it rained; it means it's started raining. 我饿了 — I've got hungry. This 了 lives at the end of the sentence and reports a change of state.

It's a different job from the completion 了 that follows a verb, even though the character is the same. 吃饭了！ is not "the meal was eaten" — it's "food's ready!"

This is the 了 that makes 我知道了 ("ah, I see now") different from 我知道 ("I know").