The Succubus has the highest attack range (can attack from 7 squares away), making her useful for putting her on the back as she has the lowest stats out of all of the Heros. Despite this, she is relatively safe from almost all sorts of damage with many units in front of her. She is often used with Shamans due to their speed boost, and it allows her to attack extremely quickly and make her a lethal Hero to the right player. Many players tend to use the Wuba's Pipe item to increase her attack speed even further so she can deal extremely larges amounts of damage .
You can find an overview about all items here.