Mages who refuse the Circle's control become apostates and live in fear of a templar's powers--the ability to dispel and resist magic. As servants of the Chantry, the templars have been the most effective means of controlling the spread and use of arcane power for centuries.
The Templar specialization is useful to any warrior. In many cases, an enemy mage is possibly the biggest threat due to their ranged damage and disables. Templar abilities help you resist these disables with the initial mental resistance bonus, and the "Mental Fortress" ability. A close range warrior is terrible for mages, especially when the "Cleanse Area" ability lets you remove disabling glyphs, and magic related stuns on your party members. When at close range, your passive ability "Righteous Strike" lets you drain mana on each hit. If you can't get within melee range, the final ability "Holy Smite" gives you access to a ranged attack useful for attacking a mage, while depleting their mana, and stunning them, all at once, while you close in.