A lot of RDD methods take a numPartitions parameter that lets you specify the number of partitions in the result. For example, groupByKey.
The DataFrame counterparts don't have a numPartitions parameter, e.g. groupBy only takes a bunch of Columns as params. I understand that the DataFrame API is supposed to be smarter and go through a LogicalPlan, and perhaps determine the number of optimal partitions for you, but sometimes you want to specify the number of partitions yourself. One such use case is when you are preparing to do a "merge" join with another dataset that is similarly partitioned with the same number of partitions.
