Analyzing the NFL preseason from a betting perspective is unlike anything else.
Unless you are seriously into breaking down the back half of NFL rosters during the offseason, you're placing bets that will be decided by players you don't know and may never hear about again. Some bettors see value in preseason lines because the oddsmakers don't know those third- and fourth-stringers either. You'll see big changes in point spreads that you'll never see in the regular season.
It has become even harder in recent years because some coaches are blowing off the preseason. They want to keep players healthy, so they'll barely play any key players, if they play them at all.