Who is Pinkeye and for what is he employed in Animal Farm? Why has this become necessary?

Pinkeye is Napoleon’s food taster because there are supposed plots to murder Napoleon. 

Napoleon was in negotiations with both neighboring farms of Frederick and Pilkington to sell a pile of lumber.  Frederick really wanted the wood but would not offer a “reasonable price” and there were rumors that his men were getting ready to storm the farm and destroy the windmill. 

In the middle of the summer the animals were alarmed to hear that three hens had come forward and confessed that, inspired by Snowball, they had entered into a plot to murder Napoleon … (Ch. 8) 

Snowball was the farm’s scapegoat.  Whatever the pigs wanted to stir the animals up about, they would say that Snowball was against them and working with the enemy.  Getting animals to falsely confess was just the next move.  Part of the propaganda campaign was to take steps to guard Napoleon’s safety. 

Four dogs guarded his bed at night, one at each corner, and a young pig named Pinkeye was given the task of tasting all his food before he ate it, lest it should be poisoned. (Ch. 8) 

The propaganda runs high.  There are rumors about Frederick and Pilkington, and Squealer tells the animals “to avoid rash actions and trust in Comrade Napoleon's strategy.”  The pigs are thrilled with Napoleon’s cunning when he sells the wood to Frederick, after appearing to sell it to Pilkington, and then horrified when he is paid with counterfeit money.  Frederick and his men storm the farm to destroy the windmill.

Pinkeye's presence is partially propaganda, because Napoleon wants to demonstrate his importance and how irreplaceable he is to Animal Farm.  He is important enough to have a guard of dogs and a food tester.  It also demonstrates the real paranoia that comes with being a tyrannical leader and having sneaky dealings with the humans.