Why Use NGNX (or any running server) to host a React web app?

I recently joined a team that deploys their main React frontend to an Nginx server running on EC2. The people who made this decision are long gone and did not leave docs explaining why they made this decision.

I am used to building my React apps and hosting them on S3 buckets since they are static files at the end of the day.

Are there any advantages or certain use cases where hosting React on a running server is favorable to S3?