With a 4°C temperature increase globally, countries representing 73% of the global population would face a 580% increase in flood risk, new study looking at river flood risk finds. In addition, 79% of the global economy would face a 500% increase in flood damages.

what is the use of such a study?

I wish this were a less common sentiment about research. Sure, applied research is incredibly important, but studies / research / science need not have immediate applications or uses. "What's the use?" The use is increasing understanding and knowledge generally; figuring out how things work; figuring out where are methods and hypotheses go wrong, are limited, can be improved, etc. etc..

To quote some roughly relevent West Wing (S3E16) relevant to the question about the uses of science and research:

[while discussing the importance of funding the superconducting supercollider - a controversial scientific project that would cost billions]
Sen. Jack Enlow, D-IL: If we can only say what benefit this thing has. No one's been able to do that.
Dr. Dalton Millgate: That's because great achievement has no road map. The X-Ray is pretty good, and so is penicillin, and neither were discovered with a practical objective in mind. I mean, when the electron was discovered in 1897, it was useless. And now we have an entire world run by electronics. Haydn and Mozart never studied the classics. They couldn't. They invented them.
Sam Seaborn: Discovery. Dr. Dalton Millgate: What?
Sam Seaborn: That's the thing that you were... Discovery is what. That's what this is used for. It's for discovery.

/r/science Thread Parent Link - ec.europa.eu