Just gonna go off topic (Sorry Justicar): Blizzard have there own cinematic team, so they use it to create trailers detailing about the storyline so when the player goes to play it, they have a basic concept of what the game is about. Also a cinematic trailer boosts sales. That's why a lot of game company do it now. Because lots of people enjoy watching them and spread the word, "Have you watched this?"
On Topic: Never played starcraft but this trailer is good