Adding a unique record ID to data has a number of use cases:
1. Identifying the original sort order of the data (which might be meaningful for various uses).
2. Generating a single column primary key when the data does not have one.
3. Creating new numeric IDs to blind data.
4. Creating new numeric IDs to replace existing alphanumeric keys that are overly large (e.g. UUIDs).
In my work we see all these use cases regularly and without that support in Tableau Prep we're not able to use it.
This idea goes with https://community.tableau.com/ideas/8715 on sorting output tin Tableau Prep.