When you create a new project, it doesn’t have any translations and all translation sets have to be created one by one. In case of a larger GlotPress installation it usually makes sense to use the same translation sets from another project. Usually a parent one.
This was the case with translate.wordpress.org. We made a project Twenty Ten for translating the new default theme in WordPress 3.0. The project was a sub-project of WordPress Development. Since all translators of the trunk version of WordPress would want to translate the new default theme too, I just synced the translation sets of Twenty Ten with those of WordPress Development using the new mass-create feature.
You can see the feature in action in this short video:
As shown in the video, you can choose which project to sync translation sets with and then preview the added/removed sets before clicking Submit.