Why was Taiwan important to the Kangxi emperor?

Taiwan was of critical importance to the Kangxi emperor primarily because it served as the stronghold for the Zheng regime, which continued to resist Qing dynasty rule.1

The Zheng family, descendants of Zheng Chenggong (Koxinga), operated as a powerful maritime force that monopolized trade across the China seas and refused to submit to the Manchu government. By maintaining their headquarters on Taiwan after taking the island from the Dutch in 1662, the Zheng family posed a persistent threat to the stability of the Qing empire.1

For years, the Qing government struggled to mount an effective attack due to a lack of naval power, resorting to the forced relocation of coastal populations to isolate the Zheng regime. In 1683, taking advantage of internal strife within the Zheng family, the Kangxi emperor ordered his troops to cross the Taiwan Strait. The subsequent surrender of the Zheng forces allowed the emperor to incorporate Taiwan into the province of Fujian, effectively neutralizing the resistance and securing the region under his power.1