The reason that you want to get Jack in the Box first is so that you can set them up in the brush where you'll be hiding at the first gank. Since they only last a minute, you'll want to set up as many as you can, to achieve maximum damage. If you have four or five set up and they walk into it, it's enough to kill them if they're far enough in without you having to even hit them once or ignite them, if they're on the squishier side. This will not always guarantee a kill, but if placed correctly, it can easily defeat squishier champions and even secure a kill on tougher champions.