Another reason to do a transition is to get the low fiber content out of your digestive system. If you have a low fiber diet, then when you start a reboot you can experience constipation. Our digestive system slows down on a reboot, and that last bit of steak and potatoes can be slow and difficult to pass. When we start a reboot, we want only high fiber content in our digestive system, so we can pass it with ease.
I personally have never experienced "detox" symptoms, and I have a high fiber diet to start with. Because of this, I do not do a transition at all, I just jump in to a reboot cold turkey, and I never have an problems doing so.
As Charron said, some people have fairly bad dietary habits, and jumping straight into a reboot can be unpleasant.
Yet another reason to do a transition - some people have never done a reboot before and don't know what to expect. On my first reboot, my mouth constantly watered. I drooled all over myself non-stop. I would wake up at 3am in a panic thinking I was going to starve to death and die. We slowly transition into a reboot to get used to the routine.