Hello,
For example, [[MediaWiki:Aboutsite]] have the default message "About {{SITENAME}}". But we have overwritten it by "ウィキペディ アについて" （translated word from "about Wikipedia").
Performance-wise it is even better, if all main messages which have {{SITENAME}} get replacements with literals. Otherwise you're adding up 5ms of page load time to each page. :)
Domas