Some of my projects are billed hourly, and even for projects that I am not actually billing on an hourly basis, there is value for me personally in keeping track of how much time is invested in them. On my PC, I use Freshbooks to keep track of it all. Freshbooks is a Web-based tool, so I can use it on the iPad just as I do on my PC.
What about when I am not connected to the Internet, though? Granted, the same issue exists with my notebook when I am on the go, but perhaps there is an app for that. Freshbooks had an iPhone app at one point. I have no idea why it doesn't exist anymore.
There are other options--like MiniBooks Lite--which uses the Freshbooks API to connect and sync with the service. It is an iPhone app--which means tiny app that only works in portrait mode--but it is better than nothing if I need to keep track of my time and don't have a Wi-Fi connection handy.
This is an area that I typically just use Quicken for on my PC as well. However, with the iPad--and its cameras--there are actually better options, like the Expensify app.
Quicken is a great tool for managing finances and tracking expenses. It lets me attach an image of a receipt for my expenses from a file, from the Windows clipboard, or directly from the scanner. It beats saving all of the receipts in a shoebox, but using the scanner is tedious.
Expensify lets me save a receipts by just taking a picture of it with the iPad camera. It then scans the receipt and automatically populates the expense details--date, amount, merchant, etc. Then, I can go back and edit it with comments, assign it to a category for better organization, or tag it to make it easier to find later.
There are obviously other business functions that can be accomplished on the iPad, and with tens of thousands of apps available there is no shortage of options. Don't take my word for it. Explore the options for yourself.
In a nutshell, I can't find anything so far that I would need to do on my PC that can't be accomplished on the iPad. I am sure such things exist--but they aren't things that I generally do so they don't impact me. As I said earlier, though, it may sometimes feel a little like you are swimming upstream and doing things the hard way when you can't just use the tools you are already familiar with.