Adding new file types makes Source Insight aware of new types of files. All the defined file types become Source Insight's file “vocabulary”. That is, when Source Insight shows you the files in a directory, it will only display files that belong to currently defined file types. In addition, when Source Insight automatically adds files to a project, it will only add files that belong to known file types. Therefore, by adding a new file type, you are expanding Source Insight's knowledge of file types.