[sunsetonsaturn]

Record, produce transcript, look for keywords, alert the puppeteer when something interesting is picked up - trade secrets, pre-shared keys, defense sector intelligence, etc.

[goodpoint]

And even record keystroke sound to extract passwords.

[hackernewds]

Only works if there's labeled data for your prior keystrokes as training data. Unless, there's some uniform manufacturing defect per key in a widely available keyboard like Macbook Air