Mostly from the angle of trying to evaluate the parts. It's silly to hack a chapter out of a work and then try to judge it in isolation. No more than judging a detail in isolation gives you a good idea of the painting from which it sprang. But considering a series book by book. . . we've all read the painful semi-endings to make a unit, and the still more painful chunks of infodump in the first chapter to make them saleable book by book. And reading a series out of order is often wrong-headed because it works like reading the chapters in a random order. Even chapters temporally mixed up are put in their out-of-order sequence for a reason. (Well, not quite, since the writer may not have been designing them for that sequence. Still, even when not, they are often better in sequence.)
This aspect, of course, is not something you see so much in the episodic series. Especially the series that is really united only by its settings. Within Discworld, I recommend reading the sequences within the setting in order -- Mort before the rest of the Death books, for instance-- but one sequence before another doesn't really matter. And Andre Norton's Witch World series are similar. Some trilogies or other sequences follow characters, but frequently it's nothing but the world.
Even an event in the life of series can work the same way. There may be romantic or other personal subplots that develop from book to book, but the central aspect is killing the Monster of the Book or solving the Crime of the Book. Those can often stand alone.
Then I ran across another aspect, trying to ponder whether the fourth book of a series really had been weaker than those before. It's possible, in these series, to tell any sort of tale. However, you may set a pattern, establishing that this is the sort of criminal mystery you are dealing with: the hero is meandering along with his life, runs across the clues, adds them, and ends up at the trial, establishing guilt and innocence. And then in the fourth or fifth book, diverge from the pattern. Which means from what the readers expect.
To be sure, they want to see new and wonderful things in the pattern, but a half -- well, maybe not, maybe a third -- the pleasure in the accumulating sequence is seeing the artful variations on the form that can be wrung out without diverging from it. It's one thing to write a book of poems of all forms. It's quite another to begin a sonnet sequence and then start to throw in terza rima -- however good it may be an example of terza rima.