The activity Generate Data Table takes one string as input and then separates it into Columns and Rows based on the separators you specify in the Formatting Options (“Column Separators” and “New Line Separator”).
But because you start with a list of strings, you need to combine them into that one long input string.
Naturally, you need to have an input string that you can easily separate again using the Formatting Options of the Generate Data Table activity. Therefore, when combining your list into a single string, you must use a custom separator that will be unique to the data.
This is why you have to specify the separator twice - once to join the list of strings, and again to split it into rows.
Hope that helps