The final shipped game generally looks better than the earliest footage, but our audiences aren't always sensitive to that when evaluating early peeks at a game. These lines aren't legal protection so much as a gesture to temper the "this game looks like shit" reaction.
By overlaying it on the frame, they ensure that if any secondary source - like a video blogger or gaming news outlet - later repackages the old footage (after newer, shinier stuff is available, or after release), that it won't be confused with gameplay in the shipped game, leading to a skewed perception of its visual quality.
On the other hand, many game companies enhance promotional material to fake certain features the product doesn't have yet. Examples are:
- Recording gameplay videos on extremely high graphic settings even the best systems can only render as a slideshow, and then fake a fluent framerate by running the game engine in slow-motion and speeding up the finished video. The assumption is that when the game is finally released, the graphic engine will be more optimized and graphics hardware will have evolved so fluent gameplay is possible on that quality level.
- Digitally enhancing screenshots and videos to add graphic effects the graphic engine doesn't do yet but which are planned to be added.
- Pre-scripting and staging gameplay events which are supposed to be procedurally generated in the final version.
However, there is always a risk that these features will never make it into the finished game because of time and budget constraints or simply because they don't work out as expected. A "Not the final version" disclaimer protects the company from false advertising claims in this case.