Suppose you have two tables. Table1 contains two field ID & NAME and 10 rows, while Table2 contains the same fields and 20 rows. Now you enter this code in script editor:
if you removed the third statement then statement 4 will throw a table not found error because statements 1 and 2 will result in one table with 30 rows. Qlik automatically treats tables with the same fields as a single table and will concatenate the results.