Segregating documentation to a subpage means that updates to the documentation don't cause large numbers of pointless template updates across the wiki, and the template itself can be protected if necessary (without protecting the documentation). That is, each time a change is made to a template, all pages that use that template are updated, which can be significant for widely used templates. Similarly, when changes are needed to a widely used template, experiment first on {{Sandbox}} so only a single change will be needed to the template itself.