As others have said, the iPad is extremely portable so it's easy to use on the couch or around the kitchen table.
We have found that the iPad tends to be a much more social device than a computer; even a laptop. I think that's because it's so easy to turn it and showthings to others (it always makes the screen right side up however you turn it) or hand it to someone else. It's also easy to have others gather around and touch something on the screen to interact, whereas a computer you pretty much have to be in front of the keyboard.
It's a great tool for school because you can quickly read (iBooks), look up facts or definitions on the web, view interactive maps (Google Earth is so nice when you can touch it), listen to music or foreign language lessons, etc. Plus there are lots of nature field guide apps (i.e., iBird Pro, Audubon guides) and household helpers like recipe apps (Paprika).
For many people an iPad can be a primary computer. Karen uses hers that way for writing and proofing SCM books with Sonya. She does have the wireless keyboard that she uses only for heavy typing.