It's awesome that Tableau Prep Builder/Conductor can publish extracts as Tableau Data Sources on Server/Online, but they aren't "true" Tableau Data Sources for every critical intent and purpose.
Ideal Tableau Data Sources feature lots of important metadata that lower the barrier of entry to analysis for casual users. This includes calculations that use aggregates (e.g. profit ratio as SUM(Profit)/SUM(Sales)); hierarchies; groups; field descriptions; setting default properties; folders; and more. When a Prep-built TDS is published, it is not possible to further organize and prepare the TDS for self-service consumption as comprehensively as one can do in Desktop—what you see in Prep is exactly what you'll get on the other side.