The ComponentURL subclass is another specialization of PortalURL that allows the creation of WebUI components URLs. ComponentURL is commonly used to trigger WebUI events from client side:

<% def componentURL = uicomponent.event(...); /*or uicomponent.url(...) */ %>
  <a href=$componentURL>Click me</a>

Normally, you should not have to deal with it as the WebUI framework has already an abstraction for managing URL known as URLBuilder. The URLBuilder implementation delegates URL creation to ComponentURL objects.