If it truly does not mean much you rest certain guys who will benefit from it and play others. You also consider key contributors/superstars to not put them at risk. You still have a 53 man roster to work with and can you bring up 3 guys from the PS for any one week. You cannot "rest all the starters", there are not enough guys to do that.
At this point the 1 seed looks difficult, we need the team with one loss to lose 2 of its final three. It is certainly possible since the games are not easy and Mahomes is injured but they can still easily win two of those. it is possible the Broncos are locked into a position in week 18 but the Chiefs need to win....whereby the Broncos rest guys and the Chiefs go all out....still too many scenarios are possible, we need it to play out more. Also, the Bills could just as likely drop from 2 to 3 or 4 if they were to lose games. They lose tie breakers to Houston and Baltimore....and maybe Pitt. So they need to keep the petal on the floor for now.