Canonical URLs tell search engines which version should receive indexing credit. They are important for query parameters, localized entries, and repeated content.
ELI5
Think of Canonical URL as the checklist that keeps the app open for real people. The preferred URL for a page when similar or duplicate versions exist.
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.