An user report:

If possible, it could be very helpful to have borders for tiles (buttons and folders), especially for tiles with a white background as when Boards are exported as a PDF there are no visible borders compared to what you could see if the tiles had a different background colour.

For printable Communication Choice Boards, it can be quite common to have borders for different choices (tiles). This can help differentiate the choices and assist the communicator with selecting the choices they would like to.