Thank you all for your replies!
"We could refer to the introduction for this parameter Alias:" - I read that definition many times and decided to ask the question because of the defenition itself: it explains WHAT is alias and HOW it can be created but NOT FOR WHAT PURPOSE it is created!
For example, in Exchange Online an alias can be used as an additional address - much like in the same manner as "An email alias is an email address that you can use to receive email without exposing your main, personal, or professional email address." - and this does make sense.
Yes, we can also make use of alias by adding it as an additional smtp address ( "You will need to check if alias@keyman .com is included in that user's email addresses, if you cannot find this, you will receive the NDR") - but in this window we can designate ANY smtp address to be the secondary email address, NOT only the alias address so, put another way, I don't understand the purpose of alias in terms of "This value identifies the recipient as an email enabled object..." - as far as I understand it is the email address that "identifies the recipients as an email enabled object" - at least in situations when the alias is not the same as the default smtp address.