If someone speaks or writes informatively, they give you many useful facts or ideas — they actually leave you knowing more. “She answered the question informatively” means the answer had substance, not just a polite shrug. It is the adverb from informative, so it describes the manner of speaking or writing. It is a fairly formal, slightly school-report word. In ordinary speech people are more likely to say “that was really useful” or “that was full of information.” Though actually, informatively is not a compliment about being friendly; a talk can be dry and still be informative. The word only claims that facts and ideas were there in a helpful amount.