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A business can rank well and still be losing people at the point of decision, because the thing they look at is the listing rather than the website. A rating of 3.8 with forty reviews reads differently from an empty profile, and neither is visible in a crawl.
Linking a listing to a site brings that data into the same place as everything else.
What gets pulled
Four things, and they are the four a local business actually argues about.
The rating and the count behind it, plus the distribution across one to five stars. An average of 4.6 from nine reviews is not the same signal as 4.6 from four hundred, and the distribution shows whether the average is real or one five-star review holding up a bad month.
The categories the listing is filed under, and whether the listing is claimed. An unclaimed profile is the single most common finding on a small business, and it is fixable the same afternoon.
Review topics, meaning the words reviewers use most. They are not sentiment analysis and they are not a score, they are a frequency count, and they are the fastest way to see that every review mentions parking and none mention the thing the website leads with.
People also search, the similar businesses Google serves, with their own ratings beside your own. That list is the competitive set whether or not the business owner agrees it is.
What you need before you start
Two values, per domain.
The business name, spelled the way it appears on the listing. It does not have to match the domain, and on a business whose trading name differs from its website that is exactly why the field exists.
The Google Maps place ID. A string that starts with ChIJ and identifies one listing uniquely. It is not the map URL, not the CID number and not the business name. Google publishes a Place ID Finder for looking one up, and it is the reliable way to get a value that works.
If you have the place ID, the business name is what the lookup searches for and the ID is what it locks on. Supplying both is the arrangement that cannot land on the wrong business.
Why nothing appears until the next run
Saving the link stores two values on the domain. The listing data is fetched during the full SEO analysis, which is the monthly job, and that is the same rule as every other measured number in the product. Nothing is invented between runs.
So the sequence is to link the profile, wait for the next full analysis, and then the panel fills with the rating, the distribution, the topics and the similar businesses.
When a link does not produce a profile
Three causes worth checking in order.
The place ID is wrong. A map URL pasted into that field will not work. It has to be the identifier itself, and Google's own finder is the way to be certain.
The name does not match the listing. The lookup searches by name, so a trading name that differs from the listing's name finds nothing. Use the name on the profile, not the name on the invoice.
The listing is not public. A permanently closed or suspended listing has nothing to pull. The panel stays empty because there is nothing there, not because the link failed.
Clearing the link removes both values and leaves everything already measured in place, which is the rule the rest of the product follows.