To really understand the significance of the main antagonists, The Reapers, it really is worth buying the first game. It is a fantastic adventure in itself, even if you just stick to the core missions. Also, an ME1 file will give you some bonuses, and recurring characters will have more resonance, particularly Garrus. If you get to know Garrus in the first game as I did, you will be affected by his transformation over the 2 years - think the difference in jack bauer from season 1 to 2, if you watch 24 that is.