The main purpose of the Row item is to be able to center your submenu content within a wider submenu panel. For example, if you are using unbounded submenus which expand to the extents of your viewport, but you want the content to be centered within a 960px area, you would wrap your submenu content in a Row item. You can set the width of the row container in the UberMenu Control Panel > Main UberMenu Configuration > Position & Layout > Submenus > Submenu Row Width
Control Panel – Row Width Setting
Centered submenu content on front end
Using Rows in the Appearance > Menus panel
Add a row as a child of your top level items and parent of the submenu content that should appear within the row to center that content.
[Row] items can also be used to separate submenu content into different rows, as an alternative to the “New Row” option. Note that a Row item is not required to start a new row, however, so this item should only be used when necessary.