Here's why it's significant:
* It was a family heirloom: The locket was a family heirloom that had belonged to Sirius' mother, Walburga.
* It held a Horcrux: Unbeknownst to Sirius and Harry at the time, the locket contained a Horcrux of Voldemort, making it a very dangerous object.
* It was meant to be a gift: Sirius intended to give the locket to Harry as a birthday present, but he was tragically killed before he could do so.
* It was later revealed to be cursed: The locket was enchanted by Voldemort, and it had a negative effect on those who wore it, including Harry and Ron.
While the locket itself wasn't directly useful in fighting Voldemort, it played a crucial role in the story, leading to Harry's possession of a Horcrux and the subsequent quest to destroy them.