Perhaps Naruto was in a situation where he had made a deal. For example, he promised to pay for the seals in exchange for some information about a missing nin or a secret technique. He values knowledge and power, and if paying for the seals was part of getting closer to achieving his goals, he would do it.
There is a possibility that it was an ex - shinobi who had retired from active duty. This person had amassed a collection of seals during their career and now made a living by selling them to other ninjas like Naruto.
Well, considering Naruto's own experiences and the variety of seals in the Naruto universe, he could sell communication seals. These would allow ninjas to send messages over long distances. He might also sell seals that are related to elemental jutsu, perhaps ones that enhance or modify fire, water, wind, earth or lightning techniques.
The seals could also be a plot device for a darker storyline. Maybe they were placed by an unknown enemy to control Naruto or use him as a weapon. In some fanfictions, the seals start to affect Naruto's mental state, causing him to have inner struggles between his normal self and the influence of the seals.
One possible reason could be to protect her. Maybe she has some special power that attracts dangerous enemies, and Naruto, being the strong and protective character he is, seals her power to keep her safe from those who might misuse it.
Well, perhaps in the fanfic, Naruto has to team up with his friends in a unique way to seal the Juubi. For example, Sasuke could use his Sharingan to help Naruto locate the weak points of the Juubi while Naruto does the actual sealing. And Sakura could use her medical ninjutsu to keep Naruto in top shape during the process.
One possible plot development could be that the village is in great danger from an enemy too powerful to defeat directly. Naruto, seeing no other way, seals himself away to act as a last - line - of - defense deterrent. If the enemy tries to completely destroy the village, Naruto's sealed power could be unleashed to counter them.
The idea of Naruto sealing the Juubi is quite common in Naruto fanfictions. If Kurotsuchi is involved, it could be that the author wants to explore a different kind of relationship. For example, Kurotsuchi could be from a different village and her interaction with Naruto during this momentous event could lead to a new alliance or at least a better understanding between their respective villages. Also, the sealing of the Juubi by Naruto is no easy feat. It would require a great deal of chakra control and knowledge of sealing jutsus. Kurotsuchi, with her own skills in earth - based techniques, might be able to contribute in unexpected ways, like creating a stable environment for the sealing to take place.
His friends from the village could be his customers. They might need seals for their various missions or personal use.
One - handed seals might allow for more complex jutsu combinations. Since the hands are freer, a ninja could potentially chain together different jutsu more seamlessly. For example, a fire - style jutsu followed immediately by a wind - style jutsu to enhance the attack. This would change the power dynamics as it gives the user more versatility in combat.
Fanfiction about 'Dark Naruto, Kyuubi, Memory Seals' can take many forms. One possibility is that the memory seals are tampered with by an outside force. This could be an enemy who wants to unleash the power of the Kyuubi and corrupt Naruto. The 'Dark Naruto' that emerges might have different abilities compared to the regular Naruto. He could be more aggressive and less hesitant to use the Kyuubi's power without the moral restraints that the original Naruto has due to the disrupted memory seals.