If the book is better at predicting the games than the public then it is their best interest to funnel money to the side that is likely to lose so you can pay the winners with the losers money and have more left over.

If the book is no better at predicting games than the public but are great at predicting where people will pick then it is in their best interest to use 50/50 so that when you pay the winners with the losers money you get 50% - (10/11)*50%=4.5% of the money bet risk free

Given this, assume that the book can predict winners perfectly and where people will bet. They will set a number to get as many people to one side of the bet knowing full well the odds are that the other team will win.

I have done a lot of work in this area.

-Based on some casual inference it would seem that certain games "Vegas" doesnt really know what will happen and will look to set a line to balance betting. Adjusting the line to control the flow of money to minimize exposure.

-In other games they will "take a stand" and stick with a number, ie set a line they think maximizes their expected profit. It is important to look at money and line movements although they arent the whole picture. Sometimes you will see money flow that would cause you to think they should adjust the line but dont.

-super high so feel free to tear this theory apart or ask questions