The Field list can be useful if you need to select the background of a form, for example to set its properties in the Property Manager, but the form is completely filled with components and no form background is available to click on, as is often the case for mobile forms. To select the form or window background in this case, you can open the Field List and check the form name at the top of the list (which will deselect any components), or if you click on any individual component, then shift-click it to deselect it, the focus will be returned to the form background and its properties will be shown in the Property Manager.
New for Omnis Studio 8.1
In the upcoming release of Omnis Studio 8.1 (which is in Beta phase), you will be able to use the Space bar to select a control in the Field List, so when the focus is in the Field List, you could use the arrow keys to navigate up and down the list and use Space bar to select a control as required. The Shift-Space keypress will allow you to select (or deselect) multiple, discontinuous controls in the list.