From SPAZ wiki
In Game Description
The Mule was once a dedicated cargo vessel, but it has recently been upgraded into a serviceable combat ship. Although still comparitively slow, its turrets and missiles allow the Mule to defend itself at any range.
Like the Tug, the Mule is a good mining and hauling ship, while still able to be decent in a fight, although it's not as maneuverable. The dual Large Utility slots mean it can serve as a recovery craft for Rez, Data and Goons on battlefields; or they can be replaced with Scanner modules for hunting cloaked ships and mines.
As of version 1.501, the Mule can also be equipped with a Huge mining Beam when converting the Main turret with the Fixed Turret Mod,making it an even better miner then before.