Well, in my point of view it just means getting more comfortable when I sit, sometimes looking more elegant.
But did you know there are some meanings behind actually doing that to your legs when you sit? It is said that when you sit with your legs crossed and towards the exit door of where you are it means you cannot wait to leave that place, in other words, that you do not want to be there/ or even that you are just sick of that place. Now, you can notice girls sitting during dates with their loved ones that they can often be seen with their legs crossed in the direction of the person that they are seated beside. It means that they are comfortable and interested in what the other person has to say.
Anyways I do believe there are things behind why we do it and how we do it, so. The information I brought to you above was told by a psychologist during a conference I attended.