Templar Tanks are one of the most difficult classes to play in The Elder Scrolls Online.
There are a lot of obstacles for Templar Tanks, the biggest one being the lack of self-healing. Templars are the only class in ESO that have no heal that is scaled on Max Health, this means you are left with heals that scales with Magicka or Stamina and they are not very effective on a Tank. This means that Templars rely heavily on Shields, this will often require the use of multiple skills for shielding such as Radiant Ward combined with Defensive Stance or Bone Shield. If you did want to heal on a Templar Tank your only options are Vigor which will heal for around 6k over 4 seconds, Honor the Dead which will give you an 6k Heal but it might also Heal someone else instead, or Radiant Aura which is a 6k Heal but its an AoE Burst Heal so it costs about 6k Magicka.
The next obstacle is the complete lack of Crowd Control, Templar Tanks are the only class with no AoE Crowd Control skill of some kind, Templar Tanks rely Heavily on using both an Ice Staff with Frost Blockade for the random Immobilizing of enemies and the extremely Magicka expensive Time Stop skill from the Psijic Order Skill Line or the Stamina expensive Turn Evil skill from the Fighters Guild Skill Line. For chaining they have to use Silver Leash which is a heavy Stamina Drain. Templar Tanks do have the benefit of Restoring Focus which returns Stamina every second even while blocking, the problem is that sustain is a big issue even with this really nice sustain skill because of the extremely expensive and weak abilities meaning you spend more resources for less effective benefits compared to every single other Tank class.
There are a few group buffs from the Templar Tank, you have Purify which is a synergy that becomes available to allies stood inside the Extended Ritual that helps them cleanse negative effects. Templar Tanks also have Radiant Aura which is an Area of Effect Minor Magickasteal which is great for add pulls meaning Magicka Damage Dealers can sustain better when dealing with add pulls.
Templar Tanks are not hugely common due to having so many obstacles and also, most groups tend to use Templar Healers negating the need for a Templar Tank. In the rare occasion you are in a Magicka group, with no other Templar then it would be useful to have a Templar Tank, most likely as an Off Tank to provide the Minor Sorcery and Minor Breach mainly.