Accredited EMC testing is expensive, scheduled weeks out, and pass-or-fail. Pre-compliance is cheap, bookable at short notice, and diagnostic. The economics are obvious once you have been through a failure, and invisible before.
What a failure actually costs
The test fee is the small part. The real cost is the slot: you fail, you fix, and the next available accredited slot is three or four weeks away. If your fix requires a board respin, add fabrication and assembly lead time. A failure discovered at final test routinely puts two months into a programme, and two months of a product not selling is usually worth more than the entire engineering budget for the project.
What to test, and when
The useful moment is when you have a working prototype in something close to the final enclosure, with real cables, and before the layout is frozen. Too early and you are testing a board that will change; too late and you cannot act on what you find.
For most connected products, four things are worth an afternoon. Radiated emissions — the usual killer, and usually the clock harmonics, the switching converter, or a cable acting as an antenna. Conducted emissions if mains powered, where the input filter either works or does not. Radiated immunity, where you find out whether your ADC readings wander or your device resets in a field. ESD, which finds the connector and enclosure gaps that a layout review never will.
The findings are usually the same handful
In my experience the recurring causes are: a switching regulator with no local filtering and a hot loop area; a crystal or high-speed clock routed near a connector; a ground plane split that turns a return path into a loop; a cable shield terminated with a pigtail rather than a 360-degree bond; and an enclosure where a display cut-out has become a slot antenna at exactly the wrong frequency.
All five are cheap to fix at pre-compliance and expensive to fix later. Three of them are free if caught before layout is frozen.
What pre-compliance does not do
It does not give you a certificate, and a pass in a pre-compliance chamber is not a guarantee of a pass in an accredited one — different chamber, different setup, different margin. Treat it as risk reduction, not assurance. What it buys you is the right to book accredited testing with a reasonable expectation of passing first time, and a list of fixes while they are still layout changes rather than tooling changes.
If you are within a month of design freeze and have not put a prototype in front of an antenna, that is the highest-value day available to your project right now.