Google XML Sitemap and Robots.txt

Ok guys, i am a beginner and am trying to understand the best way forward regarding this issue. I use a robots.txt plugin which creates a robots.txt file, easy, but i also use a plugin called google xml sitemap, which in the settings say it will create its own robots.txt file so DO NOT create one (which i already have), but my websites are found fine via google, so does the sitemap plugin just overwrite the other one and in which case i don’t have to bother using a robots.txt plugin?

thanks in advance for and clarity… 🙂