What is the importance of mod_rewrite?

the Redirect directive is part of Apache’s mod_alias, not mod_rewrite.

mod_rewrite enables “pretty” permalinks, ie: http://yourdomain.com/a-post-title/ rather than “ugly” permalinks ie:http://yourdomain.com/?p=99.

WordPress will operate just fine without mod_rewrite and pretty permalinks, it’s just not as attractive or SEO friendly.