Comparing Submit JSON to Google Forms is like comparing apples to oranges. Both products handle form submissions, but the use cases and applications between them are quite different.
Google Forms is a "form builder" where Submit JSON is a "form backend".
Developers will generally choose Submit JSON, whereas non-technical folks will generally prefer Google Forms.
Google Forms may even be the correct choice for your web project. If you do not care how the form looks or behaves, or simply need to gather some information, check it out!
But if you'd like to maintain control over your UI and UX, or do something fancy like alert yourself on Slack when a new user subscribes to your SaaS, Submit JSON is the more appropriate choice for your project.