Simple answer is to save money, reduce spam reports, get better signal-to-noise ratios in analytics, etc — basically all the same reasons you should be doing opt-in in the first place.
Like all topics, there’s a lot to learn about it, differing opinions, etc — for example, some might say not to send an re-opt-in email to an email that’s recently opened an email, others might say that the open was just automatically triggered.
To learn more, Google to start with might be “re-opt-in campaign”:
https://www.google.com/search?q=%22re-opt-in+campaign%22
