Smuxi

I am not sure what the translations projects actually do (and never heard about them). Smuxi is gettext based and thus provides pot files and needs po files for each language. How do those projects make it even simpler?

Well, not every translator is a developer and not every translator is up to learning how to work with source control systems, gettext, potfiles and installing editors just for the sake of translating. They might well spend their time doing other tasks.

Usually the rationale for translation projects is to make the work of translators as pleasant as possible. The least they provide is a web interface, so no need to install third-party software and everybody can jump straight into translating from anywhere he is, through the browser. But they also usually provide a means for translations to be reviewed, when a translator is not 100% about what he did, so we get better translations. I know Launchpad's Rosetta for one also provides a means to have context for a string, further improving translator's work.

I think with transifex you have a translator tool now which just requires frontend

Smuxi recently migrated to transifex for simpler contribution of translations, see:
http://www.transifex.net/projects/p/smuxi/c/master/