Perhaps the potion is part of a larger plot. Snape could be acting on the orders of Dumbledore (if in a scenario where Dumbledore is still alive in the fanfiction). The potion might be crucial for a mission or a battle that Harry is about to face. It could enhance Harry's magical abilities in a specific way, like giving him temporary invisibility or the power to resist mind - control spells, which would be very useful in the fight against the dark forces.
One possible reason could be that Snape is secretly trying to protect Harry. Maybe the potion has some sort of protective charm or antidote against a dark magic threat that Snape has foreseen. He may not show it openly, but his actions are driven by his long - standing, complex feelings towards Harry's mother and his sense of duty.
The potion could enhance Harry's magical powers. For example, it might make his spells stronger or more accurate. This would be useful in battles against dark wizards or in his general studies at Hogwarts.
One possible reason could be that Harry has made a significant mistake in Potions class. Maybe he has endangered himself or others during an experiment, and Snape, being strict about safety and proper procedure, sends the Howler as a very public scolding.
It could also be for some sort of magical experiment or study. Snape, being a Potions master and a very knowledgeable wizard, might need Harry in a dormant state to study something about his unique magical abilities. For example, his connection to Voldemort might behave differently when he is asleep, and Snape wants to observe this.
One possible reason could be that fanfiction often takes creative liberties that may distort the complex character of Snape that J.K. Rowling created. Snape has a very intricate backstory and personality in the original series, and fanfiction might simplify or misinterpret it.
Well, a plotline might involve Snape initially being in denial about the pregnancy. As it progresses, he could start to form an unexpected bond with the unborn child. There could also be a plot where other characters find out and either support him or try to use it against him. Maybe Voldemort discovers it and tries to manipulate Snape in a new way because of this situation.
One reason could be that he realizes Harry has a lot of potential and wants to nurture it. He sees that Harry is in a difficult position, being the target of Voldemort, and decides to help him despite their past differences.
One possible reason could be that Snape suspects Harry of something dangerous or against the school rules. Maybe he thinks Harry is involved in some secret Dark Arts activities and ties him up to prevent him from causing further trouble or escaping while he investigates.
Isolation. Snape is a lonely character. He doesn't have many friends, and he is constantly in a difficult position between the Death Eaters and the Order of the Phoenix. His feeling of being alone and not being fully understood by anyone can lead to crying in fanfiction.
It could also be due to some external factors. Maybe there are threats from dark forces that he doesn't want to involve Lily in. If he is getting deeper into the dark arts or some dangerous situations, he might choose to give up on her to protect her from the potential harm that could come her way because of his associations.
Perhaps Harry has a hunch that Snape is involved in some secret project or fight against the dark forces that requires basilisk parts. Harry, being the hero he is, decides to provide Snape with the parts to aid in this unspoken cause. It could also be a sign of Harry's growing maturity and his ability to see beyond their past differences.