The side with the lower juice has had less action on it, so the book is exposed to a loss because most of the money is coming in on one side. So you can take the book's sweetened offer, but it's at the cost of going against the movement of the line. Which is moving towards the more accurate closing number. So you get a good deal on what has been the line liked less by sharps and other line moving money. So what you gain in less vig, you lose by lowering the winning % being on the "wrong" side.
That's the common knowledge answer, anyway. You'll learn some nuiansces over time, such as there are some books that will give you better lines on the side they think is wrong, so just betting will meaning going against the professionals. It's very case dependent.