When to use it
Use it when you want to address behavior you've seen yourself. Speak for yourself and describe exactly what happened. Unnamed “people” make the other person defensive and curious about who; your own observation is fair and specific.
Example
You: “I noticed you cut Alex off twice today. I'd want to know if it were me.”
Colleague: “Oh. I didn't realize. Thanks for telling me.”
In an email or Slack message
Say this one privately, in person or on a call. In writing, direct feedback can read harsher than intended.