Why use Textile markup language?

This is a excerpt taken from Wikipedia.com

Textile is a lightweight markup language originally developed by Dean Allen and billed as a "humane Web text generator". Textile converts its marked-up text input to valid, well-formed XHTML and also inserts character entity references for apostrophes, opening and closing single and double quotation marks, ellipses and em dashes.

Textile was originally implemented in PHP, but has been translated into other programming languages including Perl, Python, Ruby, ASP, Java and C#.

What is the advantage / reason for using a markup language like textile or a similar counterpart (markdown, reStructuredText)?

Why would one write in one markup language then use PHP to transform it into another markup language like XHTML?

Why not write directly in to XHTML?

You wouldn’t write your website in it, but you may let your users use it. Textile and its equivalents are much less verbose than XHTML, so it’s easier to read and write.