Alright, let's go back to 2012-2013 vintage Dervish, who started off in casual to get a feel for writing again after like 4 years. First thing I noticed is Casual is by far the most populated forum, where not posting for a few hours gets your roleplay shoved to the 2nd or 3rd page and any half-way decent idea usually gets a boatload of applications. In short, it's very busy verging on crowded.
I also realized that Casual can mean players range from people who still should write in Free due to one liners and speed posting to people who go above and beyond the game's standards and honestly should write in Advanced. Without detailed standards in the OOC, you get a mixture of people who fall between those two spectrums and if the GM is the kind of person who takes anyone, you can have someone writing 4-5 beautiful paragraphs that account for all the other players to be replied with, "Mary walked over and said "OWO WHATS THIS""
Obviously, that scenario can be avoided if the GM enforces some rules and standards, but let's face it, between inexperience and not wanting to tell people no because you feel like an asshole, you get a mix of player skills and writing styles that clash like a glass of orange juice and toothpaste cocktail garnished with Sour Patch Kids and horse raddish. A lot of people get frustrated with that and it probably contributes to the fact most games seem to have a hard time getting out of the first two IC pages.
So this is where High-Casual comes in; suddenly you have a quick and easy label that conveys the standards you want at a glance and the promise that most players are going to be of a similar skill and standard. That's the reason I joined them, I wanted to write with other people who could do a few paragraphs and some meaningful character development. It was pretty great... then I joined an Advanced RP and realized it was basically the same thing. Haven't looked back since.
Long story short, I think High Casual has its place because let's be honest, a lot of Advanced RPs have some pretty incredible writers and post lengths that can be daunting, and if someone wants to just write 2-3 paragraphs and practice their writing in relative comfort until they figure out what they want or are comfortable with without risking joining a game where collabs are frequent and you have 15,000 word posts at times or where other players speed post and don't have depth to their posts.
It honestly could probably use it's own 4th subforum, Moderate or whatever.