There's strength in unity.
This unified strength couldn't be overcome by a solo ranker unless the latter's individuality exceeded that of a group.
Furthermore, the individuality of a ranker took into account everything from a ranker's elemental affinities, the number of spells he could use, the Abilities he had access to, their ranking technique, elemental attainments, weapon comprehensions, to the kind of gear he used for offense or defense. So when a ranker's individuality was referred to, it referred to his entire existence.
Eren had come to realize this truth in his last timeline. Unlike Eliza who had focused on individual strength from the beginning despite being part of a prominent clan, the butcher knew the importance of acting as a team.
After all, the people at the top would always look for lone wolves who stood out so that they could suppress them. So acting in the group provided Eren with the cover he needed to stay lowkey.
