I find the visit to cottagers, whose combined poverty and sickness overtakes Emma (if only momentarily), part of an ongoing theme important to understanding Emma.

Emma seems genuinely moved by the cottagers' situation, and is at her best in attending to their needs with both competence and kindness. To me it is not a mere show, nor simply an automatic response fueled by her social status:

"These are the sights, Harriet, to do one good. How trifling they make every thing else appear!—I feel now as if I could think of nothing but these poor creatures all the rest of the day; and yet, who can say how soon it may all vanish from my mind?"
…
"And really, I do not think the impression will soon be over," said Emma, as she crossed the low hedge, and tottering footstep which ended the narrow, slippery path through the cottage garden, and brought them into the lane again. "I do not think it will," stopping to look once more at all the outward wretchedness of the place, and recall the still greater within.

And yet, just moments later, they stumble on Mr. Elton, and she is instantly wrapped up in the plan to get Harriet into the parsonage (literally and figuratively)! IIRC we never hear of the poor sick cottagers again, despite Emma's genuine concern for them.

So, while the episode could be read as showing that Emma is wrapped up in romantic notions about her own extraordinary virtue, it seems to me that Austen uses it to demonstrate not only how thwarted are Emma's legitimately good qualities, but how close she is to turning in a more positive direction. It is a thread that I see running through the novel in multiple instances.

Here, Emma herself is aware that, however poignant the experience is as it is happening, she is apt to put it out of mind. Though she convinces herself that, perhaps this time, the emotion is too great to vanish, almost immediately, it does! Her status in Highbury society, the circumscribed life she leads, the blanket adoration by her family, the big fish in small pond syndrome pointed out so well in an earlier discussion on this board—are preventing her positive qualities from blossoming in authentically commendable ways.

But the episode subtly undercuts an image of Emma as merely a deluded and self-absorbed troublemaker. She does possess a sound and genuine basis for growing into sensibility and potentially even nobility of character. The manifest suffering of the poor shocks her into an experience of her own authentic benevolence. Similarly, as Laraine points out regarding Emma's self-reflection in Chapter 16, she is not beyond looking quite honestly at her own failings in less lofty concerns—once she has no choice. Indeed, it is easy to imagine Emma being quite a different person had the circumstances of her upbringing been even slightly altered.

This is where Mr. Knightley's value as a friend is most evident. He is the only person who forces Emma to look at herself, who will not be a handmaiden to Emma's self-satisfaction. Without Mr. Knightley's influence, Emma might either be forever distracted from her nobler motivations in her zeal to shore up her superiority, or would see through her self-deception only upon a lengthy series of debacles on the order of the attempted matchmaking of Harriet and Mr. Elton.