What is the purpose of a Robots.txt file, and is it necessary to include all your sitemaps in it?

What is the purpose and importance of a Robots.txt file?

I’ve seen examples of Robots.txt files in WordPress that list allow and disallow commands followed by sitemaps. Is there a reason for this order?

Additionally, if you have an XML sitemap for Google News, do you need to include both sitemaps in your Robots.txt file?