as far as i know theres no site with straddling allowed in cash games. theres no real strategic advantage to straddling, but some argue that they help to loosen your image and how loose others play. in my experience though they're generally used by players that are bored to add a bit of spice too the game by making pots bigger.
That's possibly a bit reductive
Here's a good thread from the past on the subject.
http://blondepoker.com/forum/index.php?topic=30063.0