When you have a large cube you often want different Active Directory users to see other data from the same cube.
Imagine yourself this dimension:
Each sales department has access to his own data in the facts: Sale numbers and Amount of items sold, the General Managers can see all data.
A huge benefit of SQL Server Analysis Services is that just as accessing and manipulating data is more straightforward and oriented to the business, security is also easier to align with business needs. How to implement this see this movie: