With a little more experience, I think I can answer my own question.
Removing an attribute from the grid just hides it on the report or graph. This allows you to suppress unneeded detail and create groupings that may be more appropriate to your reporting needs. Removing an attribute from the report removes it from the dataset used to create the report or graph. This may speed up processing of the report, particularly if the attribute you are removing has a large number of values.