Isolation is a key topic with tests. The core idea is that, in general, your tests should be isolated from your production environment, or even isolated from other shared environments. If tests are not isolated, then changes the tests make can impact these environments and vice versa (external changes to these environments can break tests that make assumptions). Another level of isolation is between tests. If your tests run in parallel and make changes to the same resources, then various race conditions can occur and tests can interfere with each other and cause false negatives.
This can happen if tests don't run in parallel, but neglecting to clean up test A can make changes that break test B. Another case where isolation can help is when multiple teams or developers want to test incompatible changes. If two developers make incompatible changes to a shared...