There are at least two ways to understand this question. I will try to answer both briefly.
First, this question may be asking why people in Hawai’i should care about getting sovereignty. The answer to this is that they should care because the legitimate government of Hawai’i was overthrown with the help of the United States. The Bayonet Constitution was forced on King Kalakaua by armed rebels in 1887. In 1893, Queen Liliuokalani’s government was overthrown with the help of the US military because she was going to try to write a new constitution. Five years later, the US annexed Hawai’i. Because the legitimate government was overthrown by force (this was a government that was recognized by the US), we can argue that it was illegal for the US to take possession of Hawai’i and people of Hawai’i should be concerned with regaining sovereignty over their land.
Second, this question may be asking why people in Hawai’i should be concerned (worried) about what would happen if sovereignty were granted. The main reason for concern would be that sovereignty could possibly bring about tremendous changes and conflict in the state. If sovereignty were granted, what powers would the Hawaiian people have? Who would count as Hawaiian? What would happen to land ownership? Would reparations need to be paid? If so, who would pay them and to whom would they be paid. Giving sovereignty to Hawaiians would open up these and many other questions that would bring a great deal of conflict and uncertainty to the islands. This should be a concern to the people of Hawai’i.