You need the extension if you want to stay in Thailand longer than 180 days in a row, without leaving the country. You can request such an extension only once per visit. So after 360 days you need to cross the border and get back in on the DTV terms...
If you do, you get another brand new 180 days to visit, with the option to request an extension for that visit once... after 360 days, you need to leave the country again etc.etc....
I'm not only using this infographic, but reading up from multiple sources. Please check your sources before posting information, because you're posting non-sense.