You don't trust devs to run things, to have git hooks installed, to have a clean environment, to not have uncommitted changes, to not have a diverging environment on their laptop.
Actions let you test things in multiple environments, to test them with credentials against resources devs don't have access to, to do additional things like deploys, managing version numbers, on and on
With CI, especially pull requests, you can leave longer running tests for github to take care of verifying. You can run periodic tests once a day like an hour long smoke test.
CI is guard rails against common failure modes which turn requiring everyone to follow an evolving script into something automatic nobody needs to think about much
Actions let you test things in multiple environments, to test them with credentials against resources devs don't have access to, to do additional things like deploys, managing version numbers, on and on
With CI, especially pull requests, you can leave longer running tests for github to take care of verifying. You can run periodic tests once a day like an hour long smoke test.
CI is guard rails against common failure modes which turn requiring everyone to follow an evolving script into something automatic nobody needs to think about much