It's really not complicated to comprehend...
It's very simple. It's part fad, part trend, part hype, but there is method to the madness. First off, just because there's an iPad for $1000 doesn't mean you HAVE to buy it. You can buy perfectly acceptable tablets running Android that will do just about anything you can do on an iPad for $70 USD.
As far as the mobility factor goes, a 7" tablet will fit into a larger pocket. I have a vest that has an internal pocket and my tablet fits it like a glove. It'll fit into my pants as well.
What's it good for? Plenty of things. I can take a quick note using a freebie notepad app. Works great when I have to go to work at a clients and need to monitor my hours. A few keystrokes won't kill you nor drive you up the wall. Writing a book like "War and Peace" - yeah... That will get on your nerves.
I also use it for surfing the web. Specifically, my local supermarket has online only deals and coupons that I use the tablet to access the site with. I can pick and choose the deals I want - and it also functions as a digital shopping list. I log into the store's free wifi and look up whatever I need.
It's also saved me at least $10. I buy stuff and on occasion, the digital coupons and deals don't always register or expire before their time at the store level. But I just whipped out the tablet, showed them the deal on their own website and I got the discounted price.
Keep in mind, this is a wifi only tablet that has no built in 3G or 4G which is fine and dandy. I don't have to pay an additional bill for it. It's mine, I own it free and clear.
Additionally, you can also play games - like the viral hit Angry Birds - and the various incarnations of that game.
So, yes, for light duty things, a tablet can do plenty for you. It's not that laptops aren't portable, it's just that tablets (especially the smaller 7" ones) are MORE portable. Is it a replacement for a desktop or laptop? Of course not! But it IS a handy tool for when you don't want to lug that 10 lb laptop somewhere - like grocery shopping. I still do 99.9999% of what I do with computers on my desktop, and that isn't likely to change anytime soon.