Sometimes when you receive data in Excel, or you've exported Tableau data to Excel and removed merged cells, you can have blanks in fields as shown below:
To populate these cells (especially when you've got loads of rows to fill and don't want to do it by hand), you need to select the column of the cells to be filled. Press CNT+G, Special, Blanks, OK.
In the first blank (so A3 in this case) press = then select the cell with the text (ie, A2), then CNT+ENTER. All your blank cells should be populated.
Before you do any sorting or other manipulation, copy and paste special the values of that column because you will notice that the original blank cells now have formulas in.
EDIT: In his comment below, Bruce Segal points out that you can easily do this by using the Add-In Reshaping Tool provided by Tableau (available here Installing the Tableau Add-In for Reshaping Data in Excel | Tableau Software).