Why is an e-mail address necessary?
Why does it make sense for both inviting and invited persons to enter an e-mail address and what is it used for?
For inviting persons, the e-mail address is necessary to send them the link for subsequent editing.
It may be set as mandatory for participants to provide an e-mail address. That way, the fixed appointment will then be sent to this address as an ICAL file so that the appointment can be conveniently imported into the participant's own calendar. If the option for providing an E-Mail mandatory is not chosen, no E-Mails will be sent at all to any partiicpant.
In both cases the address will never be publicly displayed, and will only be used to deliver this information.