Why is Key Attribute mandatory in Attribute View?

I am curious about the behavior of key attributes in attribute views.

First I thought it is kind of primary key for the entity Attribute View but then I did a few checks

Created two tables

1. CUST -Customer Table- It has Customer ID and Country ID

2. CTRY -Country Table- It has Country ID and Country Description

An Attribute View on top of these two tables

Simple, nothing complex

Now in Semantics

I am able to activate the attribute view and see the output

but what I do next is interesting..I made the CID as a Key Attribute, though it is not a key in the data ('in' for two rows)

But I am still able to activate the Attribute View and see the data as well. I would like everyone to try this out.

So what I don't understand is if this is allowed why is Key Attribute Mandatory?

I mean why should there be a key attribute if it does not check for Unique Constraint.