Translation:It is necessary.
I think the difference is that here, the it in English is kind of a dummy subject: when you say, for example, "It is necessary to work hard", there is no actual object that is this it; the word is just thrown in to make sure the sentence has a subject, which is necessary in English. In Esperanto, there is no such rule, at least not one that is so strictly enforced in cases like these, so you leave it out.
On the other hand, if "It sleeps", there really is a thing, an it, that is sleeping. In that case, you need to mention it in Esperanto, as well.