Sirius, being a brave and resourceful wizard, could form an alliance with other powerful wizards and witches who oppose Voldemort. Together, they could create a shield around Harry. He could also use his animagus form to spy on Voldemort's followers and gather intelligence to prevent any attacks on Harry.
Sirius could use his knowledge of the wizarding world's secrets. He might know some hidden places or spells that could keep Voldemort at bay. For example, he could put a powerful protective charm on the place where Harry is staying.
Well, at first, Sirius would be in a state of shock and panic. But then he'd have to step up. Their relationship would shift from one of equals (in a sense, as Harry was growing up) to a caretaker - cared - for dynamic. Sirius would have to learn about Harry's likes and dislikes as a baby. He'd sing lullabies to him, comfort him when he cries. This new experience would make their bond stronger in a very different way. Harry might not remember all of it when he's turned back, but there would be a subconscious change in how he feels towards Sirius, a deeper sense of security and love that stems from those baby - days when Sirius was his whole world.
Their relationship would be severely damaged. Harry would likely feel betrayed by Sirius. After all, Sirius is supposed to be one of his protectors. It would be a huge breach of trust. Sirius, on the other hand, would probably be filled with guilt if he realizes his mistake. He would have to work hard to regain Harry's trust, if that's even possible in the context of the fanfic.
In a 'Harry and Sirius join Voldemort' fanfiction, it might start with Voldemort making a very tempting offer. For Harry, it could be something related to saving his friends or family. Sirius might be lured by the idea of protecting Harry in a different way. However, once they join, they would face a moral dilemma. They would have to pretend to be on Voldemort's side while still holding onto their own values. And there would be a lot of internal conflict as they see the evil deeds happening around them.
It's popular because it offers a different take on the relationship. Sirius is a complex and beloved character, and seeing him interact with a baby Harry is both cute and full of potential for interesting stories.
Sirius influences Harry's early life by being a protector. He stands as a shield against any potential threats. He might use his Patronus, a large dog, to guard Harry at night, making Harry feel safe in his sleep.
Some might write it to explore a different side of the characters. It's a way to show how Harry and Sirius could be pushed to their limits and what they would do in extreme situations.
One reason could be Harry's impulsiveness. Harry often jumps into dangerous situations without thinking, and Sirius, being his godfather and having a protective nature, would shout at him to make him realize the risks. For instance, if Harry decides to go after a Death Eater alone.
One common theme is sacrifice. Sirius is often willing to put his own life on the line to save Harry. For example, in many fics, he battles powerful dark forces without hesitation to protect the baby.
Sirius would probably be very protective. He'd use his magic to create a safe and comfortable environment for Harry. For example, he might conjure up soft, warm bedding for the baby. He'd also try to find the best baby - wizarding supplies, like magic - infused milk bottles that ensure Harry gets all the nutrition he needs.
A third one is 'Sirius's Adventure with Baby Harry'. In this story, Sirius takes baby Harry on little adventures around the wizarding world. They visit different magical places and meet interesting characters. It's a story that emphasizes their relationship and how it grows over time.