The pixel runs in the browser. The Conversions API runs on your server. They send the same events by different routes, and each catches things the other loses. Meta's own guidance is to run both — the reason is worth understanding rather than taking on faith.
What the pixel sees and misses
The pixel captures rich browser context — page, referrer, device, browser-side identifiers — with no engineering work. It misses everything blocked by ad blockers, everything dropped by iOS tracking prevention, every event after a cookie expires, and anything that happens off the website.
In practice a browser-only setup loses 20–40% of conversions in consumer markets.
What the Conversions API sees and misses
CAPI sends events server to server, so blockers and browser restrictions do not apply. It can also send events that never touch a browser at all: a CRM stage change, a phone lead marked qualified, a refund.
What it misses is browser context. Without carefully passed user data and the fbc/fbp parameters, match rates drop and Meta cannot attribute the event to a click.
Why both, and how deduplication works
Send the same conversion from both sources with an identical event_name and event_id. Meta keeps the first one that arrives and discards the duplicate. You get the pixel's context when the browser allows it and the server's reliability when it does not.
Get the event_id wrong and you either double-count or lose events. That single field is the difference between a working setup and confident wrong numbers.
Does it actually improve performance?
Yes, and not only through reporting. More complete conversion data means Meta's optimisation has more signal to learn from. Accounts I have moved to a properly deduplicated dual setup typically see reported conversions rise 15–30% and cost per result fall as delivery improves.
The reporting change is immediate; the performance change takes a couple of weeks as the algorithm relearns.
What a correct setup includes
Both sources firing the same standard events, a shared event_id, hashed email and phone where available, fbc and fbp passed from the browser to the server, domain verification, and Aggregated Event Measurement priorities set.
If you want that built and verified, that is what Meta Conversions API implementation covers.