RESTful API - groupBy

The request body of the Query for data endpoint accepts a groupBy property which is an array of object items.  I understand the syntax and can follow along, but in my testing I haven't seen any measurable change in json returned in the response body no matter the pair of field/grouping I provide. The sortBy property seems to take the same parameters and returns data as expected. Has anybody had success using this property that they feel comfortable sharing?

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