Templars are guardians of each temple and they can give extra boost if you upgrade by buying templar skills.

Templar bonuses can be unlocked for the first time in Platinum league and you can aquire them with skydust.
With each templar you purchase for a given skill, the cost of skydust is increasing significantly. 

Templar skill % by upgrade:

• Striker - 0.25% per purchased unit
• Defense - 0.25% per purchased unit
• Troops Production - 0.5% per purchased unit
• Gold Production - 0.5% per purchased unit
• Speed - 1% per purchased unit

Purchased % units stack and there is no limit in the number of purchases you can make.