- grouping fields in a logic way;
- making better use of the space available on the page.
This works well on well on laptops, desktops, and tablets, but not so well on mobile phones. In general, you just don't want your fields to be shown in multiple columns on mobiles, as this will require users to scroll horizontally to see fields "to the left of the page", thus greatly reducing usability.
This is why, starting with version 4.10, your multi-column forms will automatically show in 1 column when accessed from a mobile phone, or any device with narrow screen.
This is done automatically for you, using an approach referred to as responsive web design. If you're upgrading from Orbeon Forms 4.9 or earlier, you'll get this out-of-the-box: you don't need to change anything in in your forms, or set any configuration setting.
For more, see our documentation on responsive design.