When used to visualize relative importance, it has the great advantage of an implied, absolute scale: you can see what's 50% without having to guess from the axis what the value is. You can even eyeball a majority, as in chart [1], although the colors of two adjacent parties happen to be very similar to my vision. You would not be able to do that with a bar chart such as [2]. The pie chart should not be your go-to chart, but it has its place.
Showing how domanant one factor is.
Showing how no factor is dominant.