Just a few years back we still did everything pretty much on a personal computer. Things have changed today. A person may have an Google Android phone, a Apple iPad running on iOS and a laptop running Microsoft's Windows 7.
Sure, you could type a document on Microsoft Word, edit it on the go in your iPad with Apple Pages and make last minute revisions on DataViz's Documents to Go. You could keep your files on your personal computer, email yourself work in progress so that you could access it from your iPad and Android phone. Or you could simply go to the cloud.
Google docs is a competent online document, spreadsheet and presentation creator which is compatible with Microsoft Office. You could type your document on your web browser, save it online, edit it from the your iPad in its browser and make last minute changes on your Android phone using the Google docs app (given the small screen your are better with the app than trying to use the browser). While Google docs has an offline viewer, it has no offline editor at the present time.
Microsoft has an online version of its popular Office Suite, MS Office Live which allows you to do the same thing.
Keeping your files in sync'ed between your personal computer, tablet and smarthpone is not too difficult since you can use Dropbox (2GB online storage with free ways to get more and paid options). Basically, files stored on your personal computer locally, will have a duplicate copy online, which can be accessed from your tablet and smartphone. Another option is Ubuntu One (5GB online storage and paid options) but without a iOS client it is only a good option for those who use Android mobile phones and tablets right now.
As we move from doing everything on a computer, to doing the same things on two, three or more devices, the cloud becomes a more compelling option.