So this would just be used for extracting the hops and leaving less residue in the beer?
Yeah, for physically separating hops from beer, but more. As anyone who brews dry hopped beers knows, delicate hop aromas can have their aromatic life dramatically shortened by sloppy splashing into your keg, bottling bucket, etc. - in short, by contact with air . My approach is that, by waiting until the beer clears after primary (2-3 weeks), then racking to a purged keg before adding this dry hop canister, I have given the dry hops a basically CO2-rich environment to give off their amazing aromas - with no escape, and with no need to rack again. I carb IPAs and APAs @ room temp for a week while I wait for the dry hopped goodness to dissolve and make friendly, then chill and enjoy.
EDIT - As I said before, the canister also doubles as a vessel for coffee beans, cocoa nibs, fruit, oak chips/cubes, etc. I add any of these flavorings into the canister, into the keg, tasting regularly (of course !), and then I can pull the canister when the flavor is exactly where I want it. No guesswork.