What is the purpose of rerunning the effects?

Strict Mode runs an extra setup+cleanup cycle for every Effect.
If the Effect has no cleanup logic, so it creates an extra connection but doesn’t destroy it. This is a hint that you’re missing a cleanup function.
Strict Mode lets you notice such mistakes early in the process. When you fix your Effect by adding a cleanup function in Strict Mode, you also fix many possible future production bugs
Without Strict Mode, it was easy to miss that your Effect needed cleanup. By running setup → cleanup → setup instead of setup for your Effect in development, Strict Mode made the missing cleanup logic more noticeable.

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What is the purpose of rerunning the effects?

Strict Mode runs an extra setup+cleanup cycle for every Effect.
If the Effect has no cleanup logic, so it creates an extra connection but doesn’t destroy it. This is a hint that you’re missing a cleanup function.
Strict Mode lets you notice such mistakes early in the process. When you fix your Effect by adding a cleanup function in Strict Mode, you also fix many possible future production bugs
Without Strict Mode, it was easy to miss that your Effect needed cleanup. By running setup → cleanup → setup instead of setup for your Effect in development, Strict Mode made the missing cleanup logic more noticeable.