Why not only the Zabbix API

If you did read the [What does it do? (and why)] you might wonder if the Zabbix API not already has function build in?
Well acknowledging events without the interface, as you can read:
https://www.zabbix.com/documentation/2.0/manual/appendix/api/event/acknowledge
yes

But the API does request a user-auth ID to register.
This can not be done by email, without including the password in the email. Or storing the credentials in the acknowledging service.
Both not very secure.

That is why repbix uses a generic "email reader zabbix user", most logical to already have a Zabbix user that also own the email account.
Then this user registers the acknowledgment.
The rest of the script will adjust the user ID directly in the database.
So Zabbix tables must stay unchanged ( acknowledges & media & events = used ).