I believe the best answers are:
- It draws readers into the argument and establishes that ideas apply to society. Jefferson first addresses the problem itself - establishing one's own identity, and governing one's own affairs. He wants everybody to pay attention to this argument and observe its political sense. Only later would he provide the context for this.
- It emphasizes that the principles involved are universal, not just American. The "self-evident truths" concern humanity, different people and nations, as well as every single individual on Earth. The fact that they "hold these truths to be self-evident" means that they, the colonists, are people who respect and protect free will because they have free will of their own, and they know how difficult it is to protect this basic right.
