There are many things that make Harry Potter so compelling, but the most obvious thing is the intricate universe. From the spells to the society of witches and wizards that secretly populate the world, the Wizarding World makes our world seem boring in comparison. And some of the most interesting things shown throughout the series are the magical items, such as the Marauder's Map or the Sword of Gryffindor. However, out of all the stand-out items, the most fascinating is the Time-Turner, wielded by Hermione Granger in the franchise's third installment, Harry Potter and the Prisoner of Azkaban.
The Time-Turner gives Harry and Hermione the ability to go back in time to save Sirius Black and Buckbeak from an unsavory fate. Hermione had been using it throughout the school year to take extra courses above her full course load, and when Dumbledore suggests they can save someone by going back, they immediately take advantage of the opportunity.
Time-Turners are definitely not a toy, and it's a testament to Hermione's mature demeanor that she even received one to use in the first place. They are heavily restricted by the Ministry of Magic because time-related magic is considered extremely unstable, with even slight breaches in time causing potentially catastrophic results. For additional stability in the Time-Turner device, the Ministry placed Hour-Reversal Charms on them that enable the device to go back in time for a maximum of five hours, which is deemed the safest allowable time. The number of times one turned the Time-Turner backward corresponds to the number of hours traveled.
Many people were worried about time-travel magic, especially during the time when Lord Voldemort was thought to be gone. If someone went back to the past and messed just one wrong thing up, the timeline where Voldemort was in power could very easily be restored. It was a huge risk, both on a personal level and on a larger scale. Therefore, the Ministry kept a tight grip on these devices, which is why Hermione receiving one is such a big honor.
The risks of using a Time-Turner means the user needs to fully operate in secret while they are in the past, lest they disturb the occurring timeline and cause a rippling butterfly effect. It doesn't help that the Time-Turner can include another person -- it can transport as many people that can fit within the necklace's chain. This is why the Ministry of Magic has literally hundreds of laws in place surrounding the use of the device.
However, despite it being a dangerous magical device, the Time-Turner can certainly be used for good when in the right hands, such as Hermione's. Not only did she use it for the mundane task of simply doing more schoolwork than the average student, but when presented with an opportunity to save someone's life, she uses it in the most responsible manner she can. No other magical item in the world of Harry Potter contains such risk yet yields such rich a reward.