Any author worth their salt is going to go to great pains over how their narrator's voice is used to inform the reader and when and where the narrator may cross the line from show to tell, or as an aside to the main action, or to impart something that could fall dangerously close to pulling the reader out of their suspended disbelief.
Jim Crace, in The Pesthouse uses just such a change in the voice of the narrator (as sparingly as possible mind you, though he's done it at least three times by page 174, if I've been counting right):
Later - indeed, for the rest of her life - she would wonder how easy it would have been to have caught up with them if she'd set her mind to it.
There is no way that she would know of the future - only our narrator knows that. But why does the narrator mention this at all? Why does the narrator want the reader to know that she will not attempt to catch the other people up?
Simply put: because those characters no longer play a role in the story. Crace, by cutting in with this interjection, removes any and all need to expound upon reasons for where they go or what they do. The reader won't be left hanging at the end of the story wondering what did happen to those dodering old fools. Crace saves time and sticks with the style he has elected to use. We stay with the main, pertinent action.
You see, whilst having made this distinction, there is another character, the protagonist, who has been missing for quite some pages, but Crace hasn't cut him out with a quick phrase - and we, the reader, are still expecting our heroine to rediscover him again. Hope still holds out.