Ah, well that certainly is SSRS generated MDX.

You are right that in this situation it the where clause appears superfluous and can be removed. However it does have a purpose which relates to the query context as subcubes and where clauses affect the cube differently. One noticable issue being subcubes do not restrict calculated measures, which may or may not be behaviour you want.

Try this - add these calculated members to your query:

WITH MEMBER [Measures].[x] AS

[Date].[Calendar].CurrentMember.Member_Caption

MEMBER [Measures].[y] AS

([Date].[Calendar].CurrentMember, [Measures].[Sales Amount])

Then try them with and without the WHERE clause (with only one date picked).

I think the Iif statement in the where clause is to prevent issues with multi-selects.

Much better write-ups and explanations here and here.