Because having a starting point for your thought process can be helpful:
"In physical cosmology, the Copernican principle, named after Nicolaus Copernicus, states that Earth is not in a central, specially favored position in the universe. More recently, the principle has been generalized to the relativistic concept that humans are not privileged observers of the universe. In this sense, it is equivalent to the mediocrity principle, with important implications for the philosophy of science."
"I'll just go ahead and throw out the definition from Wiki..." is a lousy way to grab your attention. My only reason I can get away with it is that I have a bit of a captive audience. ;)
You're not as lucky. You need to grab someone from the first sentence and make them *want* to read more.
What is your favorite "OMG... I need to read more" opening line, and what did you learn from it for crafting your own?