Do you need a knife if you have a fork?
Basically, for any task in which you would use another media device (a book, a tv, a radio, a cd player, etc.), the iPad will help out. It's not a necessity, but if you are like me, and enjoy living paperlessly, it can radically change your life.
I've got the iPad streaming TV right now while I work (mundane organizational tasks).
In a few minutes I will open up my work PDF on my iPad while I take notes on my computer, take handwritten notes, and create documents (I am a researcher). If I were using paper, I'd have several books and articles open at once, because the main document I am looking at inevitably needs a bunch of reference materials to interpret, or refers to lots of other secondary materials in footnotes. Usually, the main one goes on the iPad, and the secondary ones are open on my computer (a 13", so I can only view a couple at a time).
I also spend a lot of time with just the iPad. I take handwritten notes on the documents I read. This helps me focus on the task instead of playing around on the computer, and I find that I understand material differently if I write by hand. Afterwards, I use ScanSnap to turn my notes into PDF form, and sometimes type them into the computer if necessary.
Later on, I will go to a coffee shop and read a little on it as well. I have all of my notes, work documents, and fun reading on the iPad. So, everything I could need in just about any situation is immediately accessible. It is quite handy. I highly recommend the BookGem books stand. It is collapsible and very small. It helps when you aren't using the iPad in landscape mode (most of the time for me).
Despite my original misgivings about the device (I think Steve introduced it VERY poorly), I have found it to be a wonderful addition to my life in terms of work and relaxation. The MBP is a great device, but the screen is fixed in place, and for media consumption, it really can't compare to the flexibility of the iPad.
As a side note (this might be specific to my needs), I tend to spend a lot of time each day with Adobe Acrobat Pro. When that is creating documents, running OCR, optimizing, etc. you cannot open other PDFs. This is a real annoyance for me, and the iPad is nice, because I can still work with PDFs on it while the MBP does its work.