When should you use Selenium Grid over local execution?

Local execution is simplest when a suite is small enough to finish quickly on one machine and only needs to validate against one or two browsers a developer already has installed.

Selenium Grid becomes worthwhile once any of these apply: the suite needs true cross-browser coverage (Chrome, Firefox, Safari) that a single machine can't practically run simultaneously; the number of tests is large enough that sequential local execution takes too long for a CI feedback loop; or tests need to run against specific OS/browser version combinations that aren't available on the local development machine at all (older Safari versions on macOS, for instance).

Cloud-hosted Grid providers extend this further by offering a much larger matrix of OS/browser combinations than any organization would want to self-host and maintain, which is often the deciding factor once a team's coverage needs grow past what running Grid nodes in-house is worth maintaining.