deepeasttexas nailed it.
nothing in the game or the show really is a wasted interaction. it's hard to explain without getting too spoilery, but the timeline skips ahead here and there as they travel. the pieces of the timeline we see are important to the story. i'm sure they kill tons of runners and clickers in the in-between time, but that's not the point of the game's story.
we saw bill get shot and his care for frank with his hurried instructions. he's not just some guy who chuckles when his booby traps work. now it matters. now it's more than the game it was, because of frank, and because of frank, bill has changed. that's what makes a good story. bill consented for frank to fix the place up a bit. he later says in the episode that he wasn't scared until frank came along. he changed over the course of the episode, in a loving and caring and amazing way. so when he gets shot, we see the depth of their care for each other. bill has made contingincies for frank, because of course he did. but it also underlined how close bill and joel become. there is so much implied off screen work (including the blowjob) going on in this episode and they dont insult the viewer by pounding it home that joel and bill are close in the fashion they can be given who they each are. we get bill and frank's relationship. joel and tess at dinner. then joel and ellie finding the letter.
goddamn, it's so good.