Why "claim" the next spot for community story?

Story by hiddensalt2

So, sorry for the awkward title.

Basically, my question is this: for community story projects, like the "Write a story for a picture" thread, or even "Kelly's Blue Book", authors are supposed to "claim" the next spot.

Why?

Why do that?

It slows things down a ton. What if someone claims what inspires me? Guess what, now I'm not gonna write. I might even lose interest after waiting for a few weeks, even if the original claim flakes out. But, consider the reverse. No one claims. Say I write a story, and two other people do, too. Now there are three stories, and I bet they would all be pretty damn different. Does it really matter if we all used the same prompt? I really don't see why.

Now, this isn't in regards to anything in particular, this is just a trend I've seen that is very confusing to me because it seems to literally force people not to write. So what's up with that?