It's not 'needed' per se, it's a 'quality of life' feature, it saves you a bit of time compared to a full restart.
What I meant to say in my reply is that you should only use it, well, when you need to use it, and a first time install without *any* customization done is not it
. (Granted, it won't hurt a bit, but it's not necessary *unless* you happened to change some settings, some fonts, or some screensaver files).
To get back to your issue, you may already have answered this, but just to make sure: What exactly happens when you try to pull a book from the Archive or the Kindle Store on the device itself, without using a computer?