It would typically be used by loading data from a query, file (ReadTextFile), or worksheet (ReadWorksheet), and appending other data directly from a worksheet or from a previously named table in memory (TableName).
Append From Table – select a previously named table from the drop down list.
Append From Worksheet– select a worksheet name from the drop down list. Note that the worksheet must contain data in columns with a header row starting in cell A1.
Blog article: Using the Append Function