Several properties provide features on a per-cell basis (per-cell dropdowns, per-cell tooltips, per-cell borders). Data in the table is provided through a data property and this property only contains row indices, column IDs, and the value of that cell for that particular row index and column ID.

The row indices aren’t fixed: data can be filtered (rows removed) or sorted (rows re-ordered) or updated via the backend, and the mapping between these properties and the data can become ambiguous.
To circumvent this, the DataTable needs a concept of “row IDs”, similar to the “column IDs” that currently exist. These IDs would be supplied by the dash developer and would not affect the order of the rows. It would be similar to a “hidden column”.