Is it helpful to have a Google Site as my Google Business Profile website?

Using your GMB site as the main website for your listing is not a great idea, and here is why.

Creating a Google site from within your GMB dashboard is good idea since it is quick process. Once created, it acts as a separate site and can act as another citation source. 

However, choosing the Google site as the associated site in your GMB dashboard is not a good idea, unless you do not already have an existing website. In most cases it is always better to use your own website in that field.