UDAs With .NET

I have a question about why I would want to do this. Some examples say to create some mathematical aggregations and use stuff like STDDEV, but isn't that part of SQL?. Are there other math functions I guess aren't part of the SQL standard in the rich .NET framework. My other question is how does the optimizer know to route a query to this aggregate? Or is it there for the application to be told to get there. When would you recommend a UDA vs an Indexed View? thanks.