While setup.py has been the traditional approach for packaging and distributing Python projects, pyproject.toml offers a more modern, tool-agnostic and standardized solution that addresses many of the limitations and complexities associated with setup.py. It is the currently recommended approach since PEP518, and offers a number of advantages over the previous setup.py approach:

• Standardization: it allows project owners to define a consistent development environment by defining the development dependencies and versions
• Improved Virtual Environment Awareness: Some tools leverage pyproject.toml to better integrate with virtual environments, ensuring that packages are installed and managed correctly within isolated environments
• Future Compatibility: pyproject.toml is designed to be forward-compatible with future changes in Python packaging standards. By adopting it, projects can ensure compatibility with upcoming tools and best practices in the Python packaging ecosystem.
• Separation of Concerns: pyproject.toml separates build configuration from package metadata and installation logic, which can lead to cleaner and more maintainable project structures. This allows developers to focus on project-specific details in setup.py while relying on pyproject.toml for build configuration.

Adoption of pyproject.toml does not mean that setuptools cannot be used, and the project can be configured to continue to use setuptools as the build backend for the project. See https://packaging.python.org/en/latest/guides/modernize-setup-py-project/ for more info.