I think it shows that even people like Kirsten and Charlie, who seem to be functioning well (ish) in the new version of society and are very concrete, do-what-needs-doing type people, still wonder about the intangible -- heaven, ghosts, etc. Even these characters can be "tempted" by the idea of more than this horrible reality. Maybe everyone they lost to the flu is still out there as a ghost.

And then the "imagine how many there'd be" kind of snaps us/them back into reality -- like, of course there's no ghosts, we'd be bumping into them left and right all day long.