What is the purpose of DOCTYPE header at the start of an XSD

I have found that the xmldsig-core-schema.xsd file that I was using in my application has a header that caused HTTP request to w3.org during marshaller instantiation, I removed this header and now I'm wondering if it can have any effect in the validation of XML documents. I'm familiar with the use of DOCTYPES on HTML or XML documents, but what is the purpose of it on an XSD file? The xsd is