It just works. Publishing graphical data sets in the past was tedious (many steps to create a "portable" chart, exporting, uploading, inserting, adjusting, etc) and static (in a way that would not adapt gracefully to different screen sizes and resolutions, such as large Retina monitor versus a smartphone). However, Google Charts removes all these frictions and creates beautiful, transportable charts that look good everywhere. Also, if you need to modify the underlying data, it is made easy and straightforward.
Some advanced options that are available in desktop productivity suites have deeper options to customize almost any parameter of the chart. In particular, I miss the ability to change the attributes of a single data point (like setting a different market color, size, and/or shape). I normally do this when I want to highlight a particular section of the data.
Frequent and rapid sharing of data sets to a wide audience. No need to install third party visualizers.