Several websites now allow you to enter text that support markdown (e.g. StackOverflow, GitHub), but not all sites will let you preview what the output will look like before posting. This extension will give you the ability to preview before posting markdown text.
Github also supports markdown README files in your repositories, which makes reading README files easier. If you write your README files in a text editor (e.g. Vim, Notepad++, etc) there is no preview, thus the reason for this extension.