For legitimate email marketers these pixels are important to have because:

1. Helps to prune users who never open emails thus giving you a smaller more engaged list.

2. Helps to see if the content they are sending to users is leading to engagement so they can send better emails next time.

Yes, yes I know. Why even send emails in the first place.

All email marketing is not evil.

But legitimate marketers have businesses to run in the real world and without these little pixels they would be sending un-targeted and mostly useless marketing messages to legitimate subscribers.