There are many reasons. Some big, some small.
Big ones:
1) Because it's completely unnecessary and inconvenient to carry my computer (in backpack) with me when I can do 85% of the stuff I want/need to do from my iPad.
This applies as much to being at home as it does everywhere else. Before I had one, I would have to carry my laptop and charger with me everywhere around the house. At minimum, once in the morning and once in the evening.
My MBP used to always be besides me in bed, because doing stuff online is usually what I do before bed, as well as first thing in the morning. This has changed though (much to my girlfriends delight), as now I just leave my computer where it is and take the iPad.
Also, since it uses an iPhone charger, there is almost always going to be one around, whether it be a friends house, restaurant, bar, hotel, car... wherever, someone's always got a charger.
2) It's a completely different experience than using the computer, often times, a much better one. For example, reading Google Reader using Reeder is an exponentially better experience than reading it on my computer. It just flows better. It looks better, it's easier to use, it's more focused, and perhaps most importantly of all... for many tasks, you can work through things faster.
Smaller reasons:
- Now I can watch a Netflix on my comp with my girlfriend, while also being able to do other stuff.
- I can watch Netflix from my iPad (in the car, at a friends house, at an airport, etc).
- Games - The iPad is an amazing gaming experience. You are carrying in a small package entertainment for everybody around. I can't tell you how many times I've saved a group from boredom by playing a game of Monopoly or RISK. A ton of awesome single player games too.
- I use iTeleport, so I can pretty much do anything I could do in front of my computer. Usually this comes down to sending myself files or information from my comp when I'm away from home.
- The music applications are amazing. I spend a lot of time creating music, and my mind has been blown by what is possible on the iPad. You can truly create professional sounding music, but 100x easier and more intuitively than you can on the computer.
- Various other applications that aren't possible on a computer.
I'm sure there are a lot more that I'm forgetting.
It just packs a huge punch for the size and convenience. Almost all of the functionality of my computer, yet I can pick it up and walk out the door without a second thought. I still use my computer a lot, but much, much less than before.