Checkout is a revenue-critical path. It must be tested with real provider states, redirects, webhooks, receipts, and failure cases.
ELI5
Think of Checkout as the checklist that keeps the app open for real people. The flow where a user pays for a product, subscription, or upgrade.
In practice
Use it when a change has to survive deploys, users, incidents, analytics, or billing reality. In practice, define the owner, input, output, and failure mode before you rely on it.