How final URL expansion works
Normally you give an ad a final URL and every click lands there. With final URL expansion switched on, Google may send some clicks to a different page on the same site instead, if its models expect that page to convert better for the search or the person involved. It crawls your site, decides which pages answer which queries, and generates matching headlines to go with them.
The setting appears in campaign types built around automation rather than around keywords. You keep two levers: you can turn expansion off, and you can exclude URLs or sections of the site so they are never used. What you cannot do is see the choice being made in advance — you find out afterwards, in the reporting that shows which URLs actually received traffic.
Why it matters
On a large, well-built site it can genuinely help. A retailer with deep category pages, or a publisher with a wide library, has more useful landing pages than anybody would ever attach to ads by hand, and expansion puts them to work against long, specific searches nobody planned for.
On most service sites it does the opposite. The pages you chose are the ones with the enquiry form, the proof and the offer; the rest of the site is blog posts, policy pages, thin location pages and old announcements. Sending paid clicks there spends money on pages that were never designed to convert, and the campaign’s cost per lead rises without any keyword or bid looking wrong.
Where it goes wrong
The first problem is intent mismatch. A page can be topically correct and commercially useless — an article explaining a service is not the same as the page that sells it, and expansion cannot tell the difference between reading and buying.
The second is measurement. When traffic spreads across pages you did not nominate, comparing landing pages becomes guesswork, and any test you were running on the chosen page is diluted by traffic that never saw it. The third is control: expansion can quietly pick up your brand pages, so a campaign meant to find new customers reports cheap conversions from people who were already looking for you. Reviewing where clicks actually landed is part of any honest landing page assessment.
What to do about it
Decide it deliberately rather than leaving the default in place. If your site has a small number of strong commercial pages, switch expansion off and let the campaign do exactly what you asked. If your site is large and genuinely deep, leave it on but add exclusions first: brand pages, blog archives, careers, policy pages, checkout steps and anything not fit to receive a paid click.
Then check the landing page reporting regularly and treat unexpected URLs as a signal about your own site, not only about the setting — if Google keeps choosing a page over the one you nominated, that page may be answering the search better. In Performance Max campaigns in particular, review this alongside the channel and asset reporting, since expansion and automated placement decisions blur together in the results.