A Google Ads campaign gets built, and out of habit, someone adds utm_source=google&utm_medium=cpc to the destination URLs, the same way every other channel gets tagged in this account. It feels thorough. It’s also solving a problem Google Ads doesn’t have.
Auto-tagging is on by default for every Google Ads account: a gclid gets appended to the URL automatically, and GA4 uses it to attribute the click in more detail than a UTM ever could (keyword, ad, device, placement, matched precisely, with nobody building a link by hand). Google’s own documentation is direct about what happens if manual UTMs get layered on top of that: don’t. The two systems aren’t meant to stack. At best the UTM parameters go unused; at worst they disagree with what auto-tagging already recorded, and now there are two descriptions of the same click that don’t match.
The actual rule is narrower than “always tag” or “never tag”: it’s channel-dependent. Google Ads has gclid; nothing else does. Email, organic social, a partner’s newsletter, a QR code on a printed flyer: anywhere auto-tagging doesn’t reach, manual UTMs are still the only mechanism available. Knowing which category a given link falls into matters more than knowing the parameter syntax.