If your employer knows you are looking for other opportunities, they could stop giving you projects and start transitioning you before you are ready, or just fire you.

That's a benefit to the employer, that probably is not a benefit to you, since you would likely want to quit on your own terms.

Another quick example is knowing that your employees are researching how to unionize, and thus knowing to bring in anti-union resources.

I'm sure there are more.

The goal is to figure out if you're close to quitting so you can be preemptively fired. This hands the final key away: That while the corporation without notice or reason could fire you, you could always show the same lack of loyalty and quit without notice. The goal is to deny the human that small balance.