Wow so people insist Giriko was unnecessary...here's my counter: narratively speaking, Giriko is meant to be a parallel to Soul: as a fellow weapon, but unlike Soul who is dependent on Maka to be effective, Giriko can fight on his own. Soul vs Giriko is meant to showcase how Soul and Maka are both in the same place: that both of them are weaker without each other and can only be stronger when together, and they both need to stop feeling undeserving of each other (a huge arc in Soul and Maka's story is their journey to overcome their mutual impostor syndrome with each other) Soul being unable to match the autonomous Giriko is a callback to Giriko's fight with Justin with whom he can fight to a stalemate, here Soul can barely do much against Giriko. Enforcing that he just isn't built to be a strong autonomous weapon and that he needs Maka just as much as she needs him.
In the end, Giriko self-destructs to enforce the recurring theme of how the lost of connections (and the ensuing isolation) can have adverse effects on one's psyche: he lost Arachne, his purpose, his link to the now disbanded Arachnophobia, and now went insane...leading to his downfall.
Does Giriko's presence lead to the characters getting meaningful upgrades power-wise? If you count that the more trust a team has with each other determines the strength of the resonance, then Soul and Maka learning to trust each other even more after going through another hardship then it might be yes. Does Giriko's presence lead to the characters growing spiritually by allowing them to make mistakes and learn from it? Yes, it provided Maka the proof that only she can bring Soul to his full potential and Soul trusted her skills and treasured her above all else. Does Giriko provide anything meaningful to the overall themes of the series? About the importance of connections and how the lost of it might lead to isolation and insanity? Yes, somewhat. Giriko builds his whole life for the sole purpose to be meaningful for Arachne. He even used his own children for the sole purpose of pleasing Arachne. His only significant relationship is with Arachne. And so when she fell...he fell. Soul and Maka win because they are able to support each other enough to survive before Giriko's ultimate fate catches up to him.
The series seem to try to show that in the end it was through everyone's connections to each other, no matter how small, that the day is saved. It's not through a single super powerful being landing a single world-shattering blow to the enemy that solved the problem. But through everyone working and supporting each other in variety of ways. 'Enough dust make a mountain' as Maka said in the the final battle with the Kishin.
