Sakura in part 1 was really useless and annoying, but she changed a lot. She doesn't have any kekei genkai neither power up, but she became pretty much one of the strongest kunoichi of her generation and now she is a jounin and the best medical ninja of Konoha.
-In part 1 she made Sasuke and Naruto look like two n00bs in chakra control.
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-She did her best to protect them in the forest of death.
-Sakura protected Sasuke from Gaara when he was unconscious because of the curse seal and took the blow in his place.
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-When Sasuke left, she decided to get stronger to help Naruto in bringing him back and became Tsunade's disciple.
-She saved Kankuro's life.
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-She beated Sasori, an Akatsuki member, to find information about Orochimaru when she was just 15 years old. Chiyo recognized that Sakura didn't need her help and Sasori decided to give the information that she wanted as a prize for her deserving.
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-She saved a child and an old woman during the Pain's invasion.
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-She was the head of Konoha hospital during Pain's invasion, healed countless people and saved many lives.
- Sakura noticed the Zetsu disguised as Neji and beautifully punched him.
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-She released her seal, smashed the ground and kicked some clone's asses during the war arc.
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-Sakura saved Naruto when his ass was kicked by Madara.
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-Sakura gave all of her chakra to save Sasuke from the desert dimension and also saved Obito from acid, injuring herself to protect him.
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-She also punched a goddess in the head.
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-She healed Sasuke and Naruto and prevented them bleed till death.
That's why I can't stand when people call her useless. She may not be the best heroine, but she is not useless neither the "worst and undeveloped" character.