Devpost (as far as I understand) actually requires real software in all their hack-a-thons. See the Submissions Requirements section of the rules page, a software application is required. I believe they do this to insure submissions are real, not just slides and ideas.
RootsTech then takes the approach Matt mentioned above.
Yes but to what extent.
Is the prototype checked? Is there a code check? Etc.
If the judges aren't going to consider how much of the idea is actually functional, then maybe a quick broken prototype may suffice.