Purdue's defense has kept them in every game, it's understandable why Vegas would keep the spread close. Purdue has probably become a popular bet this year amongst other fans, not really Boiler faithful. I know my BIL was picking them often in the front half of the season, and winning.
For the game, it's really hard to tell at this point. The offense is obviously scaled way back, especially to Brohms liking. I don't think he's comfortable calling it like that. Looks like they want to give Burgess a go at WR. If he pans out, it relieves Anthrop and should open up Herdman, the latter 2 seem to be the most reliable pass catchers.
The defense should keep it close as always.