Often, in such fanfictions, Sirius may not trust Harry with certain information about his parents or the wizarding world because he believes Harry is too young to handle it. So, he keeps things from Harry, which can lead to conflicts between them. Harry may feel frustrated that Sirius doesn't see him as a capable young wizard, while Sirius is just trying to protect what he sees as Harry's innocence.
In this kind of fanfiction, this dynamic can be quite complex. On one hand, Harry could start to internalize the 'baby' treatment and act more immaturely. He might not take risks or stand up for himself as much. However, on the other hand, it could also be a form of protection for Harry. Sirius may be trying to shield him from the harsh realities of the wizarding world. This could lead to Harry having a more sheltered view of things at first, but as he matures, he could break free from this perception and become a stronger character.
Well, it could add a really strange and unexpected dynamic to the plot. It might make the story more about Bellatrix's warped sense of power and control. Maybe she's using this 'baby - like' treatment to manipulate Harry in some way, like trying to break his spirit more easily by infantilizing him.
In some cases, it can be normal, especially if she's used to taking care of you in a certain way for a long time. But if it's making you uncomfortable or hindering your growth, it's not so good.
One possible reason could be that Voldemort sees Harry as a symbol of innocence that he wants to corrupt. Just like how he enjoys toying with his enemies, treating Harry like a baby might be a way to belittle him and make him more vulnerable in his eyes.
Perhaps Draco has developed some sort of complex feelings towards Harry. He could be secretly attracted to Harry and treating him like a baby is a way to get closer to him without admitting his true feelings. Or it could be that Draco has changed as a person and now views Harry as someone who needs his support and guidance, just like one would for a baby.
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.
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.
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.