The key to see here is that reduces the liberties on the top black chain, making a forcing play inside.
Reuven: Your vars are correct! I was thinking of the difference between the position with a W stone at the marked locations and without. So just to add some completeness it's nice to evaluate the position prior to W playing this stone. Or if you feel the ko isn't as interesting the problems wording can be changed... (Either way the problems wording should be improved and my English isn't that good so please edit it ^^*)