There's no downside to creating a GMB website when you already have a website. Just be careful because when you create it GMB defaults to that as your GMB listing landing page. So you need to specify that you don't want it as your GMB landing page.
Good question. It is of great benefit to a small business that doesn't have a website. If you already have a website it seems that it can provide some SEO value. I'm doing some testing around this so I can't confirm at this time but there is a lot of buzz around the SEO value of the GMB website within the local SEO community.
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