When review requests were first rolled out, they were not very useful because there was no way to see a list of pull requests you had been requested to review. Thankfully, that deficiency has been fixed!
Now, review requests unleash a great way to manage workflow on pull requests.
When you create a pull request, you add a review request for one or more reviewers. In addition, assign the pull request to the reviewer to signal that you are awaiting their action.
When the reviewer finishes their review, they should either merge the pull request (LGTM!) or assign it back to you (or someone else) for action -- respond to a comment, answer a question, revise code, etc.
Then when you respond to feedback, you assign the pull request back to the reviewer, and the cycle repeats.
There is additional explicit signaling in the review request workflow that you can use, too.
So, improve your pull request workflow by: