I agree with @adammaxum that the primary category is most important. But the additional categories are also very important depending on the vertical.
For instance, if you are an insurance agent who offers mainly auto insurance but also write renters, life and health insurance you would absolutely want to add the renters, life and health insurance categories.
The dental vertical is another great example where adding additional categories is critical. For instance if you offer teeth whitening you would want to add the teeth whitening GMB category as a secondary category.
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