J.E. Sawyer said:
The two most salient are 1) separating the BB from the actual product means it's easier to separate the product platforms when the full game is released 2) the BB is a beta, so it will be full of bugs, possibly crashy, and generally not fully polished. It may also have wild balance issues, exploits, and GUI issues that will (one hopes) not be present in the final game. However, anecdotal data strongly suggests that people playing betas do not evaluate them as betas, but as final products. Accordingly, they trash the product's ratings and give scathing reviews. If the beta product is later "upgraded" to the final product, all of the hooting and hollering done about the beta remains part of the final game. So separating the beta product from the final product appropriately segregates comments and ratings for each one.