The idea is popular as it allows for a different perspective on Snape's character. He was often seen as a villain or at least an antagonist to Harry. In fanfiction, when he 'uses' Harry, it can be for good reasons like protecting him from a greater danger or guiding him in a way Dumbledore couldn't. This shows a more complex side of Snape and adds depth to the story.
Another reason is that it can lead to some exciting plot twists. Since Snape's true intentions were often hidden in the original books, fanfiction writers can play with the idea of him using Harry in various unexpected ways. For example, Snape could be using Harry to find a lost magical artifact that has the power to change the fate of the wizarding world, and along the way, their relationship evolves in a completely unforeseen manner.
It's popular because it creates a new dynamic. Snape and Harry had a complex relationship in the original series. Fanfiction writers like to explore how Snape, with his deep knowledge and skills, can impact Harry's journey in different ways.