A few months ago, i started a theory, Maria/Rose/Sina are not the only walls that exist, maybe there's another one... the most external wall, Annie/Berthold and Reiner comes from the city of that wall, cause, i dont really think that the humanity created that walls full of titans without know this, and i dont think that it takes less than a few hundred of years, that extension of the walls are enormous, so by force, there should be another wall
Or maybe.. There's another 3 walls with other cities... That people does not have the technology that the people inside of our know walls have (3D Gear), but they have the hability to be titans, and here comes what i think about all this..
For the first wall theory, i think those people from the externall wall wants that power to end with the titans and try to save themselves, knowing that the people inside Maria/Rose/Sina will die when the Colossal Titans wake up, so they need that power to survive
For the second theory, the other city theory.. It means this is some kind of race, one has technology and the other one has power (titan power), but they all are inside walls with colossal titans inside, one day that titans will wake up and destroy all the humans, its like a time bomb, we dont know why or when, but it will happen
No matter what theory, Ymir knows this, also Reiner/Berthold/Annie, thats why when Eren show his power for first time, Ymir thinks "Maybe there's a future inside the walls...", i think the real power of Eren its command the titans by his own feelings, this was the investigation of Grisha Yaeger (who i'm sure its dead, cause as Ymir said, everyone who transforms in titan, have eated at least one person, i'm sure Grisha was devoured by Eren, that caused his amnesia, like happened to Ymir) , this power can be feeled by all Titan users too, but they have own will, so they only ear the command, but unlike the titans, they dont feel the force of make what Eren wants
So.. Why Eren its so important?, cause this power can be used for.. Destroy the walls using the collosal titans to kill every normal titan, and then make them kill themselves
This can explain the despair of Reiner/Berthold, or why Annie cried when she lost Eren on the forest, his despair for survive, as they say, "We're not bad people!", "Now we're just mass murderers"
Also, i dont know if all of you realised this, but the titan that Ilse see in the forest before die, says "Its the people of Ymir..." "Ymir-Sama..", yes, OUR Ymir, make sure that she's one of the most important characters of all the manga, also, the Ape titan has probably the same power as Eren, but he comes from "the other wall" or "The other city", thats why Reiner/Berthold are so surprised when they see him