The general point is to take into account how much the novel actual worries about day-to-day survival or life patterns in a world that is greatly changed.
Well obviously, in a lot of post-apocalyptic settings, people are going to be starving. But not in all, some folks are better prepared, and other stories take place long enough after the initial collapse, that there has been a bit of a recovery.
One item, I never gave much thought to was people's eating patterns. For instance, will people eat breakfast? I would have thought so, but apparently they didn't use to:
Ian Mortimer on Life in the Tudor Era
Five Books Interviews, 21 March 2013
Did people not have breakfast in Tudor times?
Before about 1600 people tended not to eat breakfast. It’s in the last decades of the 16th century that breakfast became habitual. In the Middle Ages you would eat breakfast if you were going on a long journey and therefore getting up early, or if you were a worker working in a field, on a harvest day, which might be a 16 or 18 hour long working day. But on the whole most people didn’t eat breakfast. They had dinner in the late morning and then supper in the mid-to late afternoon. Those were the two meals of the day. There were a few exceptions – aristocratic families who started having ceremonial breakfasts in the 16th century and if you were ill you might have breakfast as well.
So presuming there has been a major break with today's lifestyle, it is very possible we won't be eating breakfast.But in the 16th century all the times started to get shifted around, because people increasingly worked for other people, rather than sorting out their own times of working during the day. Therefore they have to stay at work till much later, so they can’t have supper till much later, so they start eating lunch instead of dinner in the late morning and they have to have breakfast before they start. So there is a shift to a three meal pattern.