Because your application isn’t always online and even when it is, the internet is slow and unreliable.

• http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd
  https://www.w3.org/2001/datatypes.xsd
  

  Commonly accessed resources from the W3C (DTDs, schemas, etc.) are subject to a delay of several seconds. Ignore this and you could be subject to a ban of several days.

• https://cdn.docbook.org/release/xsltng/current/xslt/docbook.xsl
  

  The “current” version of the stylesheets are convenient, but what if you want more control?

• You can use resolvers to cleverly manage versioning and other aspects of your application.