Tableau Prep changes the sort of the data in each flow as required by the different tools. There are a couple of use cases for needing the option to sort the flow:
1) The output is required to be in a certain format/structure, this is more true for CSV outputs than anything else.
2) If/when the record id/index feature is implemented https://community.tableau.com/ideas/8714 then if there is a sort then we can use that as a way to generate a unique index or rank.
3) Related to #2 there are times when we might want to sample first/last N records of a flow and being able to sort that is critical.