When would you use Tableau Prep rather than preparing data in Desktop?

Prep is for shaping that has to happen repeatably and would be awkward in Desktop: unioning many files, pivoting wide data, cleaning inconsistent values, or joining sources at different grains.

It also produces a published data source that a whole team connects to, so the preparation happens once rather than each analyst doing their own version.

Desktop remains the right place for anything view-dependent — table calculations, or a ratio that depends on what is on the shelves. Prep does not support table calculations for exactly that reason.

The wider judgement: if several teams and tools need the same shaping, it probably belongs in the warehouse rather than in Prep at all.