Personally, I love strategy and video game guides, especially in book form. That's not bias, that's just fact. I have dozens of strategy guides accumulated over several decades of gaming, some of which I bought literally only as a collectible item. My bookshelf actually strains under the weight of them, and I suspect one day it may actually collapse. One of my most treasured items is my Final Fantasy 1 "strategy guide", which was actually an issue of Nintendo Power that I got when I was only 4.
They are entirely optional and non-required, but I consider there to be nothing negative about them in principle. As for why someone would purchase them in the age of wikis . . . well, remember, some children aren't allowed to use the internet. Perhaps as the years carry on, this perspective will be relaxed by increasingly internet-comfortable parents, but many parents won't let their 6 or 7 year old children use the internet unsupervised, while few would share the same concerns for a book.
I don't own these books, and at my level of knowledge in Terraria (5 years and several thousand hours of experience under my belt), I'd be unlikely to buy them at this point. But if I were just starting out, I'd be all about this sort of thing. A wiki is nice, but I'm big on my collectibles and tangible ownership.