What is the purpose of specifying a schema in a web xml?

Data Type Definition (DTD) and XML Schema Definition (XSD) - These are the two type of specifying a schema in a web xml. What is the purpose of a specifying this exactly?

I have specified this in my TestNG.xml file like this

<!DOCTYPE suite SYSTEM "http://ift.tt/19x2mI9" >

What is the purpose of testng-1.0.dtd here?

I got some information here on DTD but I really don't understand the purpose of adding this in the XML file? http://ift.tt/1FnyFxE

Any help in explaining this is much appreciated. I am building a test automation framework using Selenium WebDriver and using this in my TestNG.xml file