It’s not a new feature in Fluther. It’s just one that is easy to overlook, and that many of us may not think to use.
I think the purpose you have in mind, of wanting to take a graphical screenshot of a form entry that shows the entire message in one shot, is not a particularly common one, but I appreciate that it could be useful in some cases. (If you are on some site which doesn’t have a resize control, and you need screenshots, you could take multiple screenshots, each showing a different scroll position on the message.)
The feature can be useful here on Fluther when composing, too. I think the usual reason designers include such a control, is thinking about making it easier to compose a long message.
On web pages, this sort of control is also very commonly called a “resize” control.