What purpose does Ancient Watcher serve?

Ancient Watcher is a 2 mana 4/5 minion that can't attack.

I recently faced one in an Arena game and just let him sitting there until he randomly died to accumulated AoE damage. He seems to be a totally useless card.
Yet I heard of a deck (Watcher Druid) that actually uses him competitively and have seen him being talked about a lot. That got me wondering how people make use of him.

I could only think of 2 things that might make sense, namely Silencing him (but why would you "waste" a precious and rare silence on him?) and giving him Taunt.

Is there any other use to Ancient Watcher or is he really only useful as a cheap and valuable target for a Taunt buff?

He's mostly a situational card.

As you mentioned you can silence him, Most notably for me I typically use it in my priest deck (as priests have mass dispel and a 0 cost silence) to troll and get a monster on the field. Since it does not attack and doesn't have taunt 90% of the time players let me keep it on the field. Then the next turn I Silence, Divine Spirit, Inner Fire, and boom they've lost 10HP or more if I use multiple Divines or Power word: Shield.

Additionally, I like having Defender of Argus in my decks so I don't have to run as many taunt cards and he's not a bad addition, as sometimes when your enemy has board control you can place it down without the enemy killing it as they don't see it as a threat. If they do kill it then you've re-directed 5 Damage that could have went towards you.

I use it as an underdog-card in most situations. It has it's uses but by no means is an amazing card. I mostly keep it around for the laughs, as I play to troll not play to win :D

Uses:

• Rush: If you get him early with a (0) cost silence in a priest deck it can catch your enemy off-guard
• Board-Control: If you're losing it can be difficult to gain board control back. This card is commonly left alone as it has no immediate threat. Even if it is killed before you can silence or give it taunt it is a cheap card so it is at little loss to you (other than maybe card advantage)
• Buff: Typical usage of Silence or Taunt.
• Full-Field: This one is a long-shot, but you can use it as simply another card on the field. Some cards such as Frostwolf Warlord could benefit from having another minion on the field. Hell, you could throw him in before a Brawl just to have a slightly higher chance at killing all of your enemy's minions.
• Silence Counter: If you give it taunt it can act as a deterent for your enemy to silence away your taunt since he'll then be able to attack (As mentioned by "z -" below).

Its mostly for cheap early defense in control/stall decks. Dropping an ancient watcher or two then putting down either a Sunfury Protector or Defender of Argus (or in the case of the druid, Mark of the Wild) gives you a really cheap and effective defense. Because he also has high attack, he will be able to kill many things that tries to take him down.

Because the ancient watcher is so cheap and tough, it will often force the enemy to expend way more mana to remove it if you do manage to put taunt on him.

It also works great againts priests as insurance against mass dispel.