It looks like ~40% of Zoom users expect you to turn on your camera.

Maybe, philosophically, it's about transparency? Proving that you're not doing the dishes during a meeting (which happened to me last week -- we listened to clank, clank, clank as someone forgot to mute).

"CAMERA ON" is often best. Reasons:

If you're invisible, ~40% of your co-workers or customers may think you're not listening or inconsiderate.

Turning on the camera makes you accountable.

Facial expressions and body language transmit 55% of communication.

When a message is important, the way you convey it is equally important. It's easy to resort to email. But, if you can't be together in person, resorting to Zoom or a phone call is 2nd best. And resorting to Zoom with a camera is first best. It may not be quite as effective as face-to-face meetings, but it is far more effective than other mediums like email.

The 34X improvement number (above) was research reported in Harvard Business Review (HBR). The study's 45 participants each asked 10 strangers to complete a survey. Half of the requests were emailed while the other half were done in person. The exact wording was used for both groups. The experiment found that the face-to-face requests got 34 times more positive responses than the emails. 

Around 93 percent of communication effectiveness is determined by nonverbal cues, according to another UCLA study."

And, that’s not limited to strangers. Studies have found that even your closest friends can’t interpret your emotions in emails," according to this 2019 article about non-verbal communications in Balance Careers. This might carry over to Zoom calls where face-is-not-present.

In the case of Zoom, if your camera is turned off, you lose the facial expression aspect of communication:

Effectiveness and Building Trust

When communicating -- if it's important -- and if you want to build trusting relationships -- it's probably best to show your face.

Face-to-face works best and Zoom with the camera ON is probably 2nd best.

If an interaction is not as important, a phone call or Zoom with the camera OFF allows participants to hear your voice intonation. So those are probably 3rd and 4th best.

Lastly, when communication is transactional and feelings can't be hurt, then use Text + Picture solutions like Slack, Facebook, Instagram, Snapchat, Email, SMS.

That said, during a meeting or when a conversation is important, I try to be a Camera ON guy. While I'm not always perfect and I have to work at paying attention sometimes, I do believe Camera On builds relationships -- as effectively, professionally, and politely as possible.