#297 depicts fluid waves in a circular motion representing infinite creativity
#298 took this concept even further by making that flow pour to the center going inwards trying to show that creativity is being filled in with no indefinite limit.
You are absolutely correct - sealing the contest is a great way to ensure people don't plagarise. However, I'd imagine (as I'm no expert), this gives a greater variety - people might develop other peoples ideas using them as inspiration but it may be fruitless as the client hasn't shown a preference conversely seeing what has been done means if you come up with an idea that has is similar to something else you can ditch it. I dumped a lot of my my original sketches because I saw similarities to other ppls work.