Apologies for this being long, but if you're interested in how the lines are actually set and move, read on...
Oddsmakers don't want most to lose. And they certainly don't want most to win. They set and move the line in an attempt to encourage exactly half of the betting money on either side of the line. Here's why:
Winning payout on most spread bets is exactly 11/10. What this means is a winning bet of $11 would return $10 in winnings. So what happens to that extra $1? That's the book keeper's commission or "vig" ($1 for every $11 successfully wagered, or in percentage terms 9.0909%). Of course, they don't get any commission (vig) from the losing bets, which if they've done their job right, should exactly equal the winning bets. So ideally, their total commission on all of the money wagered is exactly half of 9.0909% or 4.545%.
Bookies want the predictability of always earning that 4.545% on all the money wagered. They don't want the unpredictability of maybe winning a large sum of money or maybe losing a large sum of money. They smartly want the for-sure 4.545% no matter what. So they set and move a line in order to encourage even money on either side of the line. Consequently, vegas lines are not a prediction of the actual point spread in a game. They are a prediction of what the average bettor thinks will be the actual point spread in a game.
Here's an example of a casino's successful opening and management of a line in a game:
Casino opens with Bama -14 over Auburn. Bets start rolling in and the casino sees that $9,500,000 has been placed on Bama to win by at least 14, and $10,000,000 has been bet on Auburn +14.
Less money is coming in on Bama -14 than on Auburn +14. So the casino moves the line to Bama -12 to encourage more Bama bets. Now an additional $10,000,000 comes in on Bama -12 vs. only $9,500,000 on Auburn +12. A total of $39,000,000 has been wagered, split between two separate lines. Let's see what happens given three different outcomes to the game:
Bama wins by 17
All Auburn bettors lose. They've lost $19,500,000 in wagers.
All Bama bettors win. Their profit on their bets of $19,500,000 is 10/11 (90.909%) or $17,727,272.73, which is paid from the losing Auburn bets.
The casino keeps the remainder (the vig) of $1,772,727.27, or exactly 4.545% of the total money wagered.
Bama wins by 13
The Auburn bettors who got the line at +12, and the Bama bettors who got the line at -14 all lose. $9,500,000 + $9,500,000 = $19,000,000 lost.
Auburn bettors who got the line at +14 and Bama bettors who got the line at -12 all win. Their profit on their bets of $20,000,000 is 10/11 (90.909%) or $18,181,818.18, which is paid from the losing bets.
The casino keeps the remainder (the vig) of $818,181.82, or exactly 2.098% of the total money wagered.
Bama wins by less than 12 (or Auburn wins outright)
All Bama bettors lose. They've lost $19,500,000 in wagers.
All Auburn bettors win. Their profit on their bets of $19,500,000 is 10/11 (90.909%) or $17,727,272.73, which is paid from the losing Bama bets.
The casino keeps the remainder (the vig) of $1,772,727.27, or exactly 4.545% of the total money wagered.
As you can see, if the casino can keep even money on either side of the line, the casino wins no matter what the outcome of the game is. The worst case scenario falls in that window between the opening line (-14) and where the line moves to (-12). In fact, that's the only window that the casino has the potential to actually lose money. Outside of that window, the casino will always make 4.545% of the total money wagered. So it's in the casino's best interest to keep that window as small as possible by accurately predicting not the outcome of the game, but where the betting money will fall, to open with a line that evenly splits the difference as much as possible, and to move the line as little as possible while still keeping even money on either side.
Casinos don't want most bettors to lose - that would eventually put them out of business. They want some happy customers and they want a dependable commission off of those customers. Hence the vig.
Incidentally, I can't find an actual sports book that's yet offering a line on the iron bowl. I've seen some predictions, but no actual sports book offers. Danny Sheridan is predicting that books will open at -17, and I see from the article linked on the previous page that the author claims LVH Superbook is offering -15.5, but I can't find an offer on LVH Superbook's website or an opening line from any other actual book. Just predictions from non-books ranging from -12 to -21. If it does eventually open as high as -17, I would expect more money to go Auburn's way, so the line should drop. I'm personally expecting somewhere around a 31-13 game.
Please correct me if anyone can link to an actual sports book that has already opened on the game.