Reading the competition rules, the prize will be awarded for algorithms that do the most societal good with the smallest estimated size of quantum computer - There is no requirement to actually execute the proposed computation. Also they give no time horizon by when the hardware should exist. So, it's very possible that no "actual uses" for quantum computers are known even after the prize has been handed out.
This is quite literally a solution in search of a problem
Reminds me of that part of Pirates of the Silicon Valley, where the people of the Homebrew Computer Club were trying to find a use for the MITS Altair. Although Quantum Computers are several magnitude orders more complex, it’s amusing to see history repeating itself.