Testing topic branches in isolation

I was recently asked about setting up different testing environments based on topic (a.k.a. feature) branches.

When a feature or bug fix was finished, it would create a new environment for it to be tested.

If it passed testing, the topic branch was merged and it would be included in the next release.

But, there's no guarantee it still works once it's been merged.

It could conflict with changes from a different topic branch and no longer work - even if it worked when it was tested in isolation on its own branch.

To ensure it still works, it will need to be tested again.

So, what is the benefit of testing it in isolation if it needs to be tested again once it's merged?

This is why I prefer continuous integration and delivery (CI/CD), as I always know all changes work together and not just in isolation.