Imagine the different colors as different browsers. At 2004 maybe blue/IE is big and through the years you'll see it grow or shrink with it's usage. I think it has the additional benefit of showing that when one takes more, the others naturally get less.
It's a reasonable choice if you want to show market share at the same time as market growth. Considering the example of browser statistics, you could imagine a graph showing the total height growing considerably over the years while the height of some browsers remains static (or even shrinks).
If the overall height grows and some browsers remain static, that means the other browsers likely didn't "steal" any of its market share, they were just better at attracting new users.
But I'm not into statistics, I might be wrong.