This Growth Rate functionality will be activated after we adding a time dimension.
The following example shows 276 Outpatients at General Surgery department in September 2014. For its Previous Period, time should be in August 2014. Actually, this Previous Period based on the smallest attributes. If you put a particular date here, for example: 2nd of September 2014, then the Previous Period will calculate the data on 2nd of August 2014. In a similar way, Previous Year is September 2013 showing 305 outpatients in General Surgery department. If you put a date here, for example: 2nd of September 2014 in pivot table, then the Previous Year showing 2nd of September 2013. So if your time dimension only has year, the results of Previous Period and Previous Year are the same.