How to pass argument into markup (renderable array)?
The docs for format_string() should clear up the confusion:


  
  @variable: Escaped to HTML using check_plain(). Use this as the default choice for anything displayed on a page on the site.
  %variable: Escaped to HTML and formatted using drupal_placeholder(), which makes it display as emphasized text.
  !variable: Inserted as is, with no sanitization or formatting. Only use this for text that has already been prepared for HTML display (for example, user-supplied text that has already been run through check_plain() previously, or is expected to contain some limited HTML tags and has already been run through filter_xss() previously).
  


So in the specific case you've provided, %wildcard is used to ensure that the value is :


  Escaped to HTML and formatted using drupal_placeholder(), which makes it display as emphasized text.


The different placeholder types are simply used in different situations, depending on what you need to do with the value, and whether or not you trust its source.

The docs for t() are also worth a read for extra context.