Gigantomakia is a powerful chess piece on the board, and if the heroes can keep control of him, it could turn the tide at least as far as keeping the League of Villains and their army at bay.

Source: https://comicbook.com/anime/news/my-hero-academia-all-for-one-betrayal-gigantomakia-shinso-brainwashed/

He joined the League of Villains as a last resort, unable to find professional or personal support and guidance to deal with his Quirk and the mental strain it inflicts on him.

Source: https://www.cbr.com/mha-tragedy-antagonists-could-have-been-heroes/

It would've served the League of Villains' agenda well, further eroding faith in the heroes and their capacity to protect people, as well as causing people to mistrust the hero schools.

Source: https://screenrant.com/one-my-hero-academia-tragedy-was-completely-ignored-by-the-story/