When there are many platforms to build awesome websites and people are already on those platforms, then why Google Sites remains the main question.
Google securely hosts the site for free. This makes it a cost-effective website wherein you don't require to pay for the hosting every year. It is absolutely free.
When you purchase a custom domain name either from Google or from a third party, you need not pay anything for mapping. The custom domain mapping is free. Eg. If you have your Google site url as sites.google.com/view/example, you can map it to your custom domain www.example.com. Google doesn't charge for mapping.
When the website is hosted on Google platform, it is as secured as its other products.
There are numerous help articles on Google communities that helps in resolving the issues 24/7. Almost all the queries have been answered in the communities. You can generate new queries as well.
No any software is required to make a website on Google Sites. Easy to handle and manage.
Google products can be integrated easily.