Flash is used to create high impact interactive sites that have the following advantages over HTML.

Flash pages load faster, can scale to the size of the viewers screen and look the same across all browsers.

To get an idea of the type of creativity that is possible with flash move your mouse over the below image. Animation created by James Teale +27(0)21 4613714

So why doesn't everybody just use flash?

You can't bookmark links on a flash page, a browsers stop button doesn't have any effect on the loading of a flash page once it's started. There aren't any navigational clues to indicate whether you have visited a portion of a site or not, no blue or purple colours to help you decide! Searching and printing a flash site isn't always easy, and lastly search engines like Google can't index your flash content.