You could try some popular fanfic websites like FanFiction.net or Archive of Our Own. They usually have a wide range of One Piece fanfics, so there's a chance you'll find this one there.
I'm not sure about this particular 'everyone watch luffy memories fanfic'. However, in a fanfic like this, it might show Luffy's memories of key battles. For example, his fight against Arlong which was very important for him to protect his friends in the village. It might also include his memories of when he first set out to sea, full of dreams and naivete.
They might gain a deeper understanding of his motivation and growth. For example, Zoro could see how Luffy's dreams have evolved over time, which would strengthen their bond even more.
One interesting concept could be exploring Luffy's forgotten memories. Maybe there are hidden traumas or past friendships that got lost in his adventures.
One reason could be the sense of community. When a lot of people are interested in something like sans memories fanfiction, it creates a shared interest. Fans can discuss these fanfictions, share their thoughts and interpretations, and it becomes a sort of social activity. Also, it can fill in the gaps in the official story that fans are eager to know more about.
One possibility is that they gain a deeper understanding of Luffy's motives. For example, they might see his memories of making the promise to Shanks and realize how important it is for him to become the Pirate King. This could strengthen their resolve to support him even more.
The relationships will likely become more profound. For instance, Zoro might see Luffy's memories of believing in his strength even when others doubted, which could make Zoro more determined to protect Luffy at all costs.
Well, it might cause a range of reactions. Some could be emotional, while others might have new insights into Luffy's past and what drives him. It could also strengthen the bonds within the Strawhats.
Usopp might be inspired to be braver. When he watches Luffy's memories, he could see how Luffy fearlessly faces powerful enemies. This would make Usopp want to overcome his own cowardice and be more like Luffy. And Robin, she would gain more insights into Luffy's kind heart. She might see how Luffy has always been inclusive and accepting of everyone, which would make her feel even more at home in the crew.