Hooks are there to expand the user base from the typical players to those who might enjoy other genres. With this I mean that a casual gamer who plays puzzle games online might not be familiar with a game in a different genre.
With the appropriate "hook" and advertising, they will learn that Game X in a completely different genre has puzzles as well and might prove to be interesting to the puzzle gamer market. With this the player might try the new game out. Hooks draw in players attention while educating on the contents of the game within the packaging.
I have also found that there are "hooks" that go completely unmentioned and are simply implied to those who find it important. Take for instance the New Mario for Wii. One hook in my eyes is that the game is much like the original Super Mario and that it is not 3D.
This was not marketed specifically, but simply implied through package images and advertising. I found it important, so I saw the hook, where others who might not find it important, don't even consider it.
All in all, the "hook" refers to a feature or enhancement in a game that is marketed or created to draw in existing player while expanding the user base to new players.