utm_content is not something a platform stamps onto your link. You add it yourself, by hand, in whatever tool builds the URL. It is the one UTM field designed to tell apart two links that already share the same utm_source, utm_medium and utm_campaign, so you can see which specific version of an ad, email or page someone clicked.
What utm_content actually marks
Say you are running one Meta campaign with two versions of the same ad, a red button and a blue button, both linking to the same landing page. utm_source, utm_medium and utm_campaign would be identical across both. utm_content is the field that carries "red_button" on one link and "blue_button" on the other, so your analytics can split them apart. The same trick works for an email with two CTAs, or a carousel ad with two slides pointing at the same offer.
What a real value looks like
There is no fixed format. It is free text you choose, usually short and lowercase with underscores or hyphens: utm_content=blue_button, utm_content=variant_b, utm_content=slide2. Analytics tools store whatever you type as a literal string. Nothing normalizes the casing for you, so Blue_Button and blue_button land as two separate rows in your reports, even though you meant one variant.
What breaks if you strip it
For the visitor, nothing. The page loads exactly the same with or without utm_content in the URL. For the marketer, stripping it collapses both variants back into one undifferentiated bucket under the same source, medium and campaign. Your split test still ran. You just lose the ability to see which side won.
When to actually set it
Set it whenever two or more links in the same campaign point to the same destination and you need to know which one got clicked: a split test, multiple CTAs in one send, multiple creatives in one ad set. Skip it when there is only one link per campaign, since utm_campaign already identifies that on its own and an extra field with nothing to distinguish just adds noise. Check the finished string with the UTM builder before you ship it, and see utm_term if you are trying to work out what that field does instead.