Liferay is much, much more than Tiles. While Tiles is a templating engine which helps with web applications and static web sites (and has been used with Struts for a long time), Liferay is both a complete product and a development platform. I like to call it an engine that runs web sites. Liferay out of the box is a content management system (complete with both local and remote staging, as well as content structures and templates), a collaborative suite (message boards, wikis, blogs), and a social network. On top of the platform, you can build any kind of site you like. It's very powerful.
The book does talk about why Liferay is better than other platforms out there. In fact, that stuff is in the first chapter, which you can download for free.
The nice thing about Liferay for architects is how freeing it is. You don't have to worry about user management anymore; Liferay handles user account management beautifully. And if you already have that in place, Liferay can connect to LDAP directories and integrates nicely with single sign-on. And it's server and database-agnostic, so you can use it with all the major app servers, from Tomcat to WebSphere, and all the major databases, from MySQL to Oracle. You can stick with the infrastructure standards you have and still use Liferay, which is probably one of the most attractive things about it to an architect who has to figure out where in the infrastructure it will fit and how it will be used.