Pure Titans make up 90% of the Titan species in AoT. They live to hunt and eat humans, despite the fact that they don't have a digestive tract and don't need to eat for survival.
Titans get all of their energy from sunlight and don't need to rely on any other energy source for survival. In fact, Titans generally don't bother hunting other animals or birds.
And yet, Pure Titans go above and beyond to chew any human they come across— only to spit them out once the human is dead.
Why Do Titans Eat People in Attack on Titan?
Titans eat humans because of a subconscious wish to regain their humanity.
A Pure Titan can only regain its humanity by consuming one of the Nine Titan shifters— a fact they’re instinctively aware of this fact, making humans their prime target.
The Nine Titan shifters are the only Titans with the ability to shift between their human and Titan form. Unlike the Pure Titans, who lose all sense of self, the Nine Titans can remain fully aware of their human minds even after transformation.
So if a Pure Titan happens to eat one of the Nine Titan shifters, it would regain the memories from its past life and will be able to switch between its human and Titan forms.
However, it's also important to remember that Pure Titans aren't necessarily aware of their target being humans. All they feel is this insatiable instinct to target them.
For example, when Rod Reiss transformed into a Titan, his vision switched, and he started seeing humans as glowing yellow dots. The light tugged on his conscious, and Reiss pursued it with an obsessive need — unaware that the glowing forms are human beings.
And thanks to their restricted awareness, like moths drawn to a flame, these Titans go after the only glowing forms in their entirely darkened vision.
1. Why Do Titans Spit Out Eaten People?
Titans can't swallow humans since they possess no digestive tract.
Titans survive on energy obtained from sunlight, making a digestive tract useless to their biology. Which is why once they consume the prey's spinal fluid, they have no use for the creature, and they spit it out.
It's like humans are chewing gums to them.
Since the sun is their primary provider of nutrition, Titans can't operate during nighttime and go into hibernation mode. This is also how the Colossal Titans inside Paradis's walls remained dormant for so long. The exterior of the wall blocked any sunlight to the Titans, weakening them until they could taste the sun again.
Humans and any other creature, on the other hand, is of zero nutritional value to the Titans. Humans are only targeted for the potential freedom they could grant the Titans.
2. What Is The Purpose Of Pure Titans?
Marley creates Pure Titans to use them as weapons against Paradis.
Any Eldian who's found guilty of crimes against Marley is punished by being turned into a Pure Titan — a fate worse than death — and is dumped on Paradis's grounds.
The idea of being turned into a mindless monster is scary enough to force the Eldians into submission. On the other hand, Eldians living in Paradis suffer greatly because of these Titans, giving Marley a petty sense of revenge. Essentially, Marley is ensuring that the Paradisians will not leave the walls because of the threat of Titans.
This is what they mean by killing two birds with one stone.
To create a Pure Titan, an Eldian is injected with the spinal fluid of a Titan, which triggers an instant transformation. If any Eldian in Marley is declared guilty of a crime, or supporting a rebellion, this is the execution they face.
Grisha Yeager's first wife, Dina, was executed this way. After being caught along with a network of spies, she was exiled and injected with the Titan fluid. Years later, Dina would eat Carla, Eren's mother, and trigger events that would lead to Eren becoming a Titan Shifter.
Why Does Marley Attack The Island In Particular?
Marley knows that Paradis will lose good soldiers when fighting Pure Titans, significantly weakening their military strength and leaving them vulnerable against an attack. This also prevents Paradisians from escaping the walls and keeps rebellions from happening.
Marley claims what they're doing is okay since, according to their history, Eldians are "demons."
This is far from the truth, though. Marley has long wanted to take over Paradis's rich natural resources for itself. And after learning of 'The Great Rumbling,' they've also been trying to gain the Founder Titan. This has led them to sanction infiltration missions aimed at stealing the Titan and declaring war on the island.