User logins to Zabbix are a lot of hassle for some use cases. The utility of a pretty dashboard is much higher if you can throw it at managers easily.

The fine people over at SquaredUp phrase it like this:

"Open Access dashboards are perfect for embedding them in user portals like Sharepoint, pushing them to wall monitors or using them as high-level reports for managers."

https://scomsupport.squaredup.com/dashboarding/sharing-dashboards-with-anyone-open-access

They originally implemented it by having the server render bitmaps, and later made a better version that would ask for login when you clicked on things.