I just draw the thumbnail and try to focus on the composition, flow of the panels, speech bubble placement and so on. Then I'll take a photo and import the thumbnail into a digital page that is the size of my comic page, and resize the thumbnail to be big enough. Then I can draw and start focusing on the actual art, knowing I have a composition that works.
To me they're really important, because it's easier to redraw a thumbnail than to put a lot of time into drawing a page, only to realize that something about the composition isn't working.