I thought there was already a way to edit certain documents
There's a basic Word and Excel editor, good for 2003 formats but not 2007+. Both come with a file system app like Air sharing so you can open documents on it. The suite also comes with an email client so you can edit files that have been emailed to you. That is, of course, an entirely distinct email client from the mail app that must download all of your emails a second time to access the file.
You can easily carry files on an iPhone - Air Sharing for one.
It's not that easy nor useful, actually. You run the app and it acts like a server, giving you an IP address and port. You then create a new network connection to access it. Of course, if you bring it to a comptuer you don't have the rights to create the network connection on, you'll have to use a browser and upload your files using an HTML form, which also means you can't create or modify directories. Once on the device, all you can do is view the files - no emailing, printing, editing, nor managing directories or renaming files.
Apps are sandboxed and with good reason. You can have your own user file system, but it's within the app. This helps the user know where their files are kept
How is that a good reason? I have no problem knowing where my files are kept on my desktop. All files are organized as the user wants which any program can access. Each individual program has a unique function and is suited for certain types of files.
The sandbox style is just ludicrous. The word processing app isn't just a word processor. It's a word processor + file system app + email client, just like any other document-based app would have to be. When you get an email with both a word document and an excel spreadsheet, you have to download the same email in each particular app (for a total of 3 separate times). Of course, any files you sync to Air Sharing won't be available to open in any editor. You have to make sure you sync your word documents with your word processing app, your excel files with your spreadsheet app, your photos with your pho... err... with iTunes, miscellaneous files with your file sharing app...
You can get to do sync in many To Do apps - pick whichever one you want.
...your to-dos with your To-do app, etc.
How is this system anything short of insane?