Suppose you're stuck in an elevator (or a dentist's waiting room, or a train station, or wherever) with a group of strangers and you think a little conversation would make the experience more pleasant, so you pipe up with what you hope is a brilliant opening line: "According to Webster's dictionary...."
I've already stopped listening. You might catch my attention with an erudite reference to etymology from the Oxford English Dictionary, but that's not what (some of) my students do: someone somewhere has told them that it's a great idea to start an essay with "According to Webster's dictionary," which makes me want to lie down.
It wouldn't be so bad if they were defining terms I don't already know or illuminating some little-known origin of a word, but no: students who start with dictionary definitions almost always define common words that any reasonable person ought to know, offering no new insight. I have read papers opening with the dictionary definition of house and poetry and, today, science.
Imagine that you're on a blind date with the person of your dreams and you feel the need to quote the dictionary definition of the word house. That's fine if the person of your dreams is learning English and has requested your assistance understanding connotations of words; otherwise, you're about to go home alone.
I know what house means! Every conceivable audience for the paper knows what house means! If the dictionary definition doesn't tell me something new and unexpected about the meaning of house, what's the point?
I know what the point is: filling space on the page. But here's the thing: text that merely fills space without adding to a reader's understanding of the topic is not fulfilling the primary purpose of the paper and is therefore unlikely to rise to the top of the grading scale. So whoever is telling students that it's a great idea to start with "According to Webster's dictionary" needs to just stop right now.
And if you don't know what stop means--well, you know where to look.