“Simple” will just return the groupings you have defined.  Rollup will add subtotal lines that rollup the values from the aggregation.  If you only have one grouping, it will just add one extra line to the model.  But if you two or three groupings,  each level will have its own subtotal lines. 

Unfortunately this is not a very useful feature, because we do not provide any ability to show that this extra row is a subtotal. 

But now you know…