Flash keeps the line as a vector and downloads it to the Flash player. It's anti-aliased so graphics look beautiful. It's scaleable so it doesn't matter what the size is of the browser window. It allows a different level of graphic design. There's interactivity - anything can become a button or be animated.
Motion is native to it.
It allows dynamic sites. I have yet to see a faster 2D vector plug-in with as much optimization. That means less waiting time for good graphics, and the user doesn't wait for the download. Recently, Macromedia released Generator, which personalizes and automates Web graphics and dynamically creates Flash. It also offers database integration and Java support.