@weizhang One benefit of RStudio Cloud, specific to using it for contest submissions, is that it provides a fully reproducible environment to run your app. What you see is what others see as well, which is a step above providing just the code as that doesn't ensure same environment, package versions, etc. easily.
Another reason is that I think it makes it easier for others, especially those who might be new to Shiny, to learn from the contest submissions. There is just something really nice about being able to launch the app and play around with the code directly!