At the start of the first session, everyone pays 1-barter and goes on to live the 1-barter lifestyle.
Imagine that there are three (or more) qualities of life available. The first is the typical quality of life for your Apocalypse World, where the PCs are, how most people around them live. What do they eat, drink, wear? Where do they sleep?
If the player pays 1-barter at the beginning of the session, that's the quality of life her character's had and can generally expect. The same as most people around her.
-- "Lifestyles", AW2E p. 151
And that's part of the purpose of the first session, isn't it? To walk through just what that 1-barter provides for you, and maybe who gets that barter and in what fractions? "Let's follow the characters around for a day and get to know them. Cool?"
Why do it explicitly, instead of just saying it's handled for the first session only and giving people less starting barter?
Exceptions are weird and it's better to fail safe than fail deadly. So in the opposite case, where everybody starts down 1-barter and it's assumed the first session is taken care of, everybody still has that Barter section on their playbooks saying "at the beginning of the session". What if somebody brings that up and the MC can't find the right counter in the rules? How many playbooks only start out with 2-barter, would start out with 1-barter in the revised case, and head into next session flat broke? (The Angel, the Chopper, the Gunlugger, and the Skinner, for reference. 4 out of 11.)
Everybody losing something means everybody feels a little pressure. Those 4 out of 11 feel it the most, of course, which is why that setup section goes on to say:
You can also assure them that their starting barter is based on the assumption that they won't work any gigs during the first session. However, you and they might want to look ahead to when they'll need some jingle, and start laying the groundwork for their gigs now.
But everybody just spent some barter and knows that every time they sit down to play they'll be shelling out more just to keep going. So everybody, except maybe the establishments of the Hardholder and Maestro'd, is going to be looking out in that first session for who's going to pay them and why.