Why does this line of code make photo albums appear?

Question

In a theme I built from scratch I'm using the WP Photo Album Plus plugin. Per some books I've been using as guides, I need to include this code in the header in order to make comments work in a blog:

<?php if ( is_singular() ) wp_enqueue_script ( 'comment-reply'); wp_head() ; ?>

I've been including this line in the header of all my custom themes, whether the site includes a blog or not (my sites are all business sites, some with blogs included but none are primarily or only blogs). This site didn't have a blog so I removed that line of code. Doing that caused the photo albums to stop appearing on the pages. The shortcodes appeared as text instead. The albums were still in the admin and the plugin was still there, and everything else still worked.

Can anyone explain the connection?

Thanks!