Prashant Sharma answer
.tdsx file, this saves your datasource in it in encrypted format & when you connect your workbook with .tdsx you will get the data & you do not need your datasource file.
Also, you are asking about why we needed .twbx file. ".twbx" file is a packaged workbook, it contains the .tdsx & what you have created into your workbook but .tds & .tdsx are just the datasources (these files will not give you what you have developed in Tableau). I hope this may help you!