Speed.
That's a huge part of it. Let's say I want to quickly look something up, like when to change the clocks back this fall. On my iPod Touch or iPhone, I can wake it from standby, launch my browser, punch the question into the Google bar and have an answer in probably 15 seconds. No PC can match that. You're looking at probably 2+ min for the average computer starting from Sleep mode, or standby, waking it, opening browser, navigating etc. Someone smarter than me can explain the differences in PC hard drives vs. tablet flash memory or whatever, but the point is, they are super fast. Zero lag time.
Apps add even more speed and flexibility. Let's say I want to know the weather forecast. Whip out iProduct, unlock, launch Weather Channel app, have forecast in about 5 seconds. Email? Same thing! One button push and you are in your Inbox, 5 seconds flat. Again, to boot up your computer and navigate on your web browser to find that takes much longer. Having the ability to run a single program at a tap of the finger to get very specific information is very very convenient.
I love my iPod touch and iPhone. I wish I could justify getting an iPad too. They are awesome to play with, and for most people's general emailing, browsing, reading, they are definitely adequate. Great for computer illiterate people too. "OK Grandma, when you want the weather, push the blue Weather Channel picture. When you want news, push the NPR picture. If you want to email me, push the envelope picture."
They don't completely eliminate the need for PCs, but they are super handy.