What credit does inclusion in "Who is Who" book add to the researchers and academicians?
They are trying to sell their books and associated vanity products. They know that their primary market for it, is people included in it.
Personally, I'd take an academic's inclusion in it to be a possible indication of gullibility.
If you've got the time, the Forbes article on the Marquis Who's Who series, "Hall of Lame", is worth a read.
How much is this website credible? Does anyone (including academic employers) give credit to those listed there (email says, inclusion in Who's Who in the World is a sign of true success. The book is a global reference source relied upon by universities, libraries, corporations, and governments around the world).
I will restrict to "academic employers" -- that is the part of you question which is on-topic for this site. Answer: no academic employer I know will give you any credit for this. Having this "distinction" on your CV would, in the context of seeking an academic job, be only negative: not very negative, but it would make you look slightly naive. The gist of this prior answer of mine (thanks to @xLeitix for calling attention to the question) certainly does apply here and explains this in more detail.
Registering into the system needs to fill a form that contains almost every single piece of information about me, including my parents, education, work, wife, certification, political/social activities and many more which is quite scary to me. How can individuals rely on this particular website and share their information. Does inclusion in this particular website gives so much credit that people lose privacy?
To the best of my knowledge, this is not a phishing scam, as you seem to be worried about. What they want to do is print books, which have lots of names together with information about those names. The more information you provide, the more material for potential inclusion they have. Some people actually take the length of their entry in these books as a point of pride (I seem to recall, well over 20 years ago, Harlan Ellison boasting that his entry in one of these things was longer than Ronald Reagan's, or something like that). This is not a new millennium thing. Go back 30 or more years, before the days of prevalent identity theft, and look at their request with those more innocent eyes. None of the information you listed is truly sensitive or confidential; it's just very detailed biographical information.
Of course, my answer to the first point renders the answer to the second point academic, but I wanted to be fair to them. One more thing: for academics, I disagree completely with what they say about wikipedia: it has quite stringent inclusion criteria, and wikipedia is a "global reference" source in the sense that organizations like this one have been dreaming about for the better part of a century but has never quite come to pass. It is also a free reference source, which is a key point that I hope that any academic would appreciate and value.
These things are essentially a vanity publishing scam. Being included typically means that you have $100 to burn and poor consumer skills. Don't touch it with a 10 foot pole.