Hi Shreepad,
I think it is related to the HANA engine to define the query access pattern when the attribute view is used in the JOIN criteria. The Key attributes may be helpful in deciding the join condition like which table and associated column should be used for the left side of the join and which one on the right. The attribute view would be joined to the transaction table in Analytic view and the column from the attribute view will then be the driver column for the join. HANA anyway redefine the join condition based on the column size and in some cases may re-align the tables, but there has to be a starting condition for the join.
So I think the Key attribute definition will help in establishing join rules.
I don't think it would enforce any data consistency check as it is a view and not persistent object. Hence the unique constraint may not be enforced.
Regards,
Ravi