For me, 'free' allows people to 'assume' what is happening. You rely more on one another to already know and accept what is happening outside the short quick posts. In some cases, you don't ever explain anything going on around you. It's more "two characters together" than it is "two characters and the world around them".

Where as advance, which I prefer, actually makes people 'set the scene'. We do not go in blind and assuming we understand one another and what's going on at all times. We have to detail out our characters, the setting, etc. It helps build a bigger world that can be seen by both minds and not just assumed by both. We play multiple people and dig into who they are as a person inside the RP. Instead of it being "Two Characters" it's more like 'A World'.

Like many advanced roleplayers, I started at what is considered 'free'. It's natural to, when you start, not grasp the concept of higher levels of 'literacy' on roleplaying sites, or not feel it's important or needed. However, like many, as we got older and learned more from those around us, we begin seeing the joy of 'details' and deeper 'collaboration'. We suddenly see ourselves creating a world.

As an author of actual novels, I see a difference (quite a huge one) between writing a story and collaborative writing. In a story, I know everything that will happen at all times because I'm doing it all myself. When I write a advance roleplay, there are surprises and things happen that I didn't know would. It changes the dynamic of the flow of the roleplay. We work off one another to build a story together where we never know what might happen next (excluding those who actually enjoy pre-planning), where as if we wrote a story alone, we know everything that will happen and nothing really changes. Instead of creating a world alone, I am creating a world with someone else.

^.^ Everyone is free to roleplay as they like. No judgement. ^.^

Also, to people who think advance writers either don't have time to post or unable to post frequently, trust me, that is not the case.
I write at 121+ words a minute using TWO fingers (excluding the shift key with my thumbs). 
Many advance writers, while true, may be lacking in time, many others can throw out 10+ posts a day when they are sitting down because they are fast typists and they can quickly work off what they are given.
Advance doesn't mean 'slow' or 'lack of time', it just means we like detail and depth in our stories. Hehe.