Grouping items is useful if you have two or more content boxes and one might grow as a result of merge data or API data of different lengths. For example, you may have an API text box or a text box with merge data (perhaps with your message text), followed by a signature box. If the message is very long, it might otherwise overlap signature box. When you group the two boxes, it will keep them together but they will not overlap.
You would create the grouping by clicking the first box, then shift+left click on the second box, and then click the Group icon in the upper left. You can ungroup them by clicking to Ungroup. When two boxes are grouped their content cannot be edited until they are ungrouped.