As you might guess from their name, adverbial pronouns are caught between two worlds: they are pronouns in the sense that they replace nouns, and at the same time they are adverbs representing a place, a quantity, or the object of a proposition. French has two adverbial pronouns: en and y.
French has two words for each of the following: year, day, morning, and evening, and they cause no end of difficulties for non-native speakers. Why is it that English can get away with one word when French needs two? The answer lies in how you look at these temporal markers.