Wrecker is a first eramedium-sized creature. While not very strong or long-lived, its useful feature is the ability to temporarily establish a summoning aura, in which reinforcements are summoned without summoning sickness.
Wrecker is a very important unit in early T1 battles. When engaging in open fights his ability is the key. Not even Frost T1 can counter Wrecker + Scavenger + Makeshift Tower when applied his ability correctly. Against Shadow T1 simply add Strikers and always drop in a few Sunstriders when too many M units and Towers are on scene. In base assault, calling out a full health Sunderer will always get some of your opponent's attention.