If you understand the principles behind website templates you will immediately see the huge benefits to WordPress template systems. Basically it is designed to hookup automated template ideas to a system via which a client who knows nothing about anything can access your site (behind password protection obviously) and update and manage the website that you leave him.
Group Sites or Social Network Sites. There are usually dedicated sites for different groups depending on what they are doing. Social Networks are much broader, but groups within them like scrapmoms, scrapsisters, and so many more usually share their ideas and projects. It is up to you whether you will join any group if you want to get more details and interact with the people within the group, but you will get templates in there.