configuration (csrf, x-frame, etc), central logging, central cookie handling, central device-detection, central session handling, central exception tracking, monitoring, analytics, app heath-checks, infra-specific uncaught exception handling, CDN awareness … … you generally wouldn't know that unless you read the README.
intention of ever opening them up has lead to better documentation, discoverability, ownership, responsibility, code-quality, efficacy, etc. By emulating practices we've seen and used in the open-source community, we've seen tremendous growth from both seasoned developers and new hires alike. By simply adopting open principles, PayPal is a measurably better place to work.