The reward for the "Homecoming" quest was a major boon for Fang Zheng.
Most importantly, as long as Fang Zheng completed the mission, this plane would become a Free World like Dark Soul and When the Cicadas Cry. Once the plane was a Free World, it meant that Fang Zheng could enter and exit this world at will without waiting for cooldowns.
Moreover, upon completing this mission, Fang Zheng's "Heart of the Swarm" Skill would no longer be exclusive to the Soulstone, but would be fixed to his person. This would save Fang Zheng the trouble of having to switch to Kerrigan each time he controlled the Zerg. Although Kerrigan's Soulstone was quite useful, there were many instances where Fang Zheng needed the abilities of other Soulstones in battle. If he was limited to only using Kerrigan while controlling the Swarm, then his strategic options would also be restricted.