There could be several reasons. Maybe the author needed a break for personal or health reasons, or there were creative challenges that led to the halt.
The manga stopped possibly due to the author's health issues or creative burnout. There might have been personal circumstances that prevented the continuation.
They could watch the development of new nen abilities. Maybe some characters watch as Ging discovers new techniques or as Hisoka creates his unique and deadly attacks. Also, in fanfiction, they might watch events related to the Phantom Troupe. For instance, the characters could be observing how the Troupe members interact with each other during a heist or when they are facing off against hunters.
It could be that the manga's popularity and the potential for future story developments convinced the publishers not to cancel it. Additionally, the depth and complexity of the story might have made it a valuable asset in the manga market.
They might be watching for strategic reasons. For example, if they are about to face a powerful opponent, they could watch their previous battles to figure out their weaknesses. Like if a hunter is going to fight Hisoka, they would watch his fights to see how he moves and what his tells are.
The Hunter x Hunter manga is unfortunately not complete. The author has faced various issues that have led to intermittent breaks and an unresolved storyline.