Thief is not necessarily pro-Town, but the Mafia usually has no need for a Thief instead of a Role Cop or Roleblocker. It usually only appears in games where roles each have some form of unique flavor.
Exactly how the Thief works varies from game to game...
The stolen items tend to reflect the players they were stolen from in flavor. For instance, a Thief would be able to steal a musical instrument from a minstrel.
Some moderators take this one step further - if the stolen item is requisite for the target role's abilities, the Thief can permanently Roleblock its target by taking away the item it needs. For instance, a magician cannot perform any Night actions if its book gets stolen.
Some moderators take this one step further than that - if the stolen item was the source of its previous owner's power, the Thief can use the abilities granted by the item. For instance, if the magician gets its book stolen, the Thief will be able to cast spells from the book as if they were the magician.
Use and Power
Thief is usually a flavorful throw-in when it is used. While it has the potential to expose falseclaims if it steals an item that is contradictory to the falseclaim, most mods will give scum safeclaims that would happen to explain why a Thief may get any result they may receive. In games where it can steal abilities from players, it is somewhat useful simply due to the reactions they may get from the newly depowered victims.
Beyond that, it doesn't appear to have much power...