Pretty much what you said, in a roleplay environment you have total freedom to do whatever you want. There are no restrictions; your imagination is the limit - barring an RP's specific tastes. 

Writing stories is like playing single player on a video game; sure it's fun, but after a while, you realise that all your characters are constricted by how [i]you[/i] think - no matter how hard you try to instil yourself with a foreign thought pattern. Kinda like how bots in video games always follow the same logic, no matter how good they are, and they become predictable. 

RPing is like going back to the main menu, and hitting the multiplayer button. You come up against characters and writing styles your own brain would never have been able to conceive if it only practised against itself, just like in video games you come across players that employ tactics and skills you'd of never encountered if you were stuck on single player ;)  

Then you take those skills you learnt from the pros on multiplayer, and kick single player to the curb with them. :D