A site-wide notice is good for important wiki-related announcements, for example. Whenever you edit MediaWiki:Sitenotice, make sure to increment the number in MediaWiki:Sitenotice id. If the number was 3, you would replace it with 4 and so on. This ensures that all your users will see the notice.
There is a separate notice for anonymous (i.e. non-logged-in users), MediaWiki:Anonnotice. If it has been defined, then it'll be shown to anonymous users instead of MediaWiki:Sitenotice. If MediaWiki:Anonnotice doesn't exist on the wiki, then MediaWiki:Sitenotice will be shown to anonymous users too.