You are able to edit your theme directly from browser, so you don't need to have a connection with FTP. It can be a huge benefit if you are a developer and your customers are submitting many small tasks, which are not hard to solve, but a time consuming to connect with their store each time and modify files. With Theme Editor you can just login and change necessary files. The process, which took several minutes, takes a few seconds.
That was the main reason for creating this plugin. In our case it shorten the small issues resolving time.