Banded rows are used by default. But Banded columns need to be selected manually. If you select both, the display becomes cluttered. Here is a situation where using ONLY Banded Columns is useful.
Look at this data below. It contains lots of text in each column and the text is going beyond the column width. Banded columns make the data more readable by showing the demarcation between where one column ends and where the next one starts! That’s it.
While we are at it, why don’t you look at all options available under Table tools in Excel.