A text version makes your message less sensitive to spam. You don't really have to do it for your readers, because almost everyone reads your nicely formatted HTML version. That is why a text version is automatically loaded in Spotler. Saves you time again.
But if you still want to format it yourself, this is possible via the Settings of your profile menu under the Settings tab.
Click on the Edit button and a pop-up will appear:
Choose manual if you want to be able to edit the text version of a message yourself