In a nested axis, you can repeat all the outer dimension row or column headers so that there are no blank cells in the nested row axis.
To do so, select the Repeat Row Headers or Repeat Column Headers.
This feature can be beneficial if you subsequently apply Microsoft Office Excel filtering or export the report sheet to a tab or comma delimited file. These two options are not selected by default since reports are more readable without the row headers repeated.
For the following report, the Repeat Row Headers is not selected:

For the same report, the Repeat Row Headers option is selected:

If you need to improve refresh perfomances, you can select the sub-option Use Excel Cell References below the Repeat Row Headers option.
When selecting this option, inside a block, the first occurrence of a header is identified as an EPMOlapMember; the other occurrences of the header use Microsoft Excel cell references and references the cell that contains the first occurrence of the header.