I have pets who like to sit on my lap, which makes the business of using a laptop/netbook a bit awkward, particularly as they just see the keyboard as a nice place to sit.
My laptop gets quite warm while in use, particularly the underside, so having it sat in my lap can be quite uncomfortable, especially in warmer weather.
So device that generates less heat and can be controlled from a touch screen would be handy. Yes, my smartphone can do all of that, but the screen is a bit small for general browsing.
I'm not sure what your problem is. Yes other devices can provide similar functionality, but why should the computing arena be different to everywhere else?
We have compact cameras for those who just want point-and-shoot, SLRs for those who want a lot of control and bridge cameras in the middle that provide a mix of easy use and advanced features.
We have a huge variety of cars for all sorts of uses and requirements, but I don't see anyone asking why Ford make the Focus when the Fiesta and Mondeo provide the same functionality.
A netbook doesn't do anything a laptop can't do, but they're the right device for enough people to buy them. I don't think it's a legal requirement to have a new device provide unique functionality,
If you can't think of a use you'd put an iPad to then I'd suggest you don't buy one.
For me, these tablet machines replicate the functions of a handful of gadgets that I currently use and carry with me regularly, so the advantage would be in having just one device. I tried a netbook and found the form unsuitable in enough situations to make me ditch it.