Around the time of the 3rd preseason game, there will be a small flurry of trades among NFL teams, as franchises try to bolster roster depth, cover for injuries, and get some value for players who are unlikely to survive the roster cut.
The principle at play for the franchise front office is usually this: if you’re gonna cut a player anyway, you might as well try to get some value back for him, and a low round draft pick is better than nothing at all.
Occasionally two teams swap players, but more often it’s a player for a draft pick.