3 reasons....

1. To increase the number of different types of MOBs in the game
2. As a teaser of things to come
and 3. to create natural boundaries and keep you out of places where there is no reason to go yet (i.e. Sandgate, Scorpion Keep, Gelmorra Ruins, The Lance, etc...) for lack of content. Of course you can still get there if you're careful...