There are two reasons why you might want or need to group a number of arguments together, so that they appear in Excel as a single argument.
There is a limit to how many arguments you can pass to an Excel add-in function, and a function with too many arguments will not work. In this case, you must select some of your arguments and group them, creating a compressed group.
The user interface may be easier to use if there are fewer arguments. For example, you may have a group of columns which are closely related, such as a column of dates and a column of payment amounts for each date. In this case, it may make sense to group your the two columns as a two-dimensional array, by creating a column group.
You can use the XLL+ Function Wizard to create groups of arguments. The wizard will generate code that splits the single Excel argument into its constituent parts, and checks that all required constituents are present and correct.