I've found that I end up putting "high casual" somewhere in nearly all of the RPs I run as GM, and for me it's to determine depth rather than length - and (as others have said in this thread) to open up Casual players to what is, pretty much, an Advanced RP. The thing is, I hate setting up "minimum post lengths" above a paragraph or two, because often times I'm busy and will break my own rule. However, what I require in all of the things I GM is depth of Character and Plot. I'd prefer two well thought-out, story-driving paragraphs over ten paragraphs of 'fluff'. High Casual (for me) also implies that some may want (and are encouraged) to write five or six or seven paragraphs in a post, and can, while others can write a consistent (meaningful) two paragraphs, and both can be accepted just the same.