To begin with I am using the tablet right now to read the thread and to post a reply. I have used a laptop in a docking station for work and also have had a desktop at home. These computers, as others have noted, have not been replaced by tablets. Tablets have been another option available for content consumption and on a limited basis for content creation. The key is portability and ease of use.
I have found personally that by using apps like Goodreader, I can load pages of documents and PDFs to read or carry and employ the tablet like a large thing ring notebook. As I move about I am carrying in one tablet, newspapers, magazines, email, articles downloaded, reports and correspondence. I go to meetings with clients and carry a tablet rather than a briefcase. Even in content creation I receive draft documents and can quickly review and edit in many office type apps. I don't wait to boot up the tablet as I do with the laptop and I do not have to be carrying around a at times heavy or heavier laptop. I do not have to open up the tablet into two halves with a keyboard. Instead I hold this wonderful piece of equipment in one hand - portability, quick accessibility, convenience.
Is the tablet a necessary addition - no. You can live without it. However if you do buy one more often than not you will find you are or will use it quite a bit. Example is my wife who is somewhat computer illiterate. She could have been an "absorber" of content from cyberspace but was not one to sit in front of desktop for hours or hold a laptop. The tablet has opened up content and the cyberspace to her. I do believe the ease of access to the NY Times, for instance, has led to more and deeper reading of the paper than ever obtained with the hard paper copy. She reads more magazines, uses the Internet more, just consumes more information and data.
Steve Jobs conceptualized a content consumption device that was not bound to a desk or required booting and using a large frame like a laptop. Something you could sit with in the living room. I can add carrying with you on trains and planes, holding in a restaurant or a Starbucks. The fact that it has found uses in business such as a holder of business information, that it has or can substitute for a notebook or briefcase of items, that apps hav developed to assist in content creation, so if you primarily found the tablet useful for most of your tasks, you did not need to lug a laptop around just to do some creation - these may not have been anticipated but yet certainly explains the growing use and desirability of obtaining a tablet and working it into the type of equipment to rely upon.
So finally when I travel my laptop often goes into the suitcase. I keep it at the hotel if I have taken expecting to do more heavy content creation. But when I am out and about I have transferred what I need to this lightweight tablet and that is what I carry.

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