A newsletter goes out with two links to the same landing page: one on the hero image, one in the P.S. line at the bottom. Same source, same medium, same campaign. Three weeks later someone asks which one actually drove the signups, and there’s no way to answer: both links look identical in every report GA4 has.
That’s exactly what utm_content is for. Of the five classic UTM parameters, it’s the one built specifically to separate two links that would otherwise be indistinguishable (two creative variants, two placements in the same email, two calls to action pointing at the same page). Skip it, and GA4 has no way to know the two were ever different in the first place.
It rarely gets filled in, because unlike a missing utm_source or utm_medium, nothing looks obviously wrong without it. The campaign total is still there, still accurate. It’s only the follow-up question (which specific link, which specific variant) that turns out to be unanswerable, and by the time anyone asks it, the campaign’s already over.