Usefully is the adverb from useful. It tells you that something was done in a way that really helped, or that a thing is being put to good use rather than sitting there doing nothing. "The money was usefully spent" means the money did some real good. "She usefully pointed out that the shop closes at six" means her comment actually helped us. There is a second, very common pattern where usefully sits in front of "can" or "could" or "be": "This tool can usefully be used for cutting paper too" — here it means "it works well for that as well". Writers also drop "usefully" into a sentence to praise a small helpful act, often with a slightly formal or written feel: "The report usefully lists all the dates in one place." You will meet this word much more in writing than in daily talk. In everyday speech people are more likely to say "it really helped" or "that was helpful".