Making your home screen look pretty is not a requirement. Putting things on it is.
Even if you don't actively theme your phone, you do actively lay out your home screen. Layouts are the way you arrange the apps and widgets on your home screen. We don't talk about them nearly as much when theming as we should. In fact, I try to avoid specific layouts in my themes so they are more accessible to users of varying tastes. But layouts are important, whether you intend to theme your phone or not.
Some people like their home screen to have as little on it as possible. Some people want everything on the home screen they possibly can squeeze in there. Some people are a little of column A and a little of column B, because we believe that the space on our home screen is meant to be used, but we don't need/want to cover up every inch of it. No matter which camp you fall in, all of us can benefit from seeing the differences and the benefits of each approach.
Heavy home screens
These are without a doubt the most prevalent layouts in the mobile space, in part because these layouts look a lot like how Windows Phone and iOS home screens look: a vast sea of app icons and (maybe) widgets. For users who have recently migrated from one of these platforms, or users on their first smartphone, this busy layout is a taste of the familiar, like the desktop they save everything to at home.
Because everything important is in one place, these users spend little time digging through apps they don't frequently use in their app drawer. It keeps some users focused on the apps they need to use, rather than getting sucked in by an app that catches their eye in the drawer. If there are widgets here, they're usually a clock, some settings toggles, and maybe a weather widget.
Despite the time-consuming look of these home screens, these layouts are usually the quickest to come together: drag everything you need to the home screen and maybe sort them. However, busy home screens also don't leave much space open for gestures beyond a double-tap.
These layouts may look chaotic to the untrained eye, but to the user who set them, everything has a place and they know where that place is. When theming a heavy home screen like these, stick to simple wallpapers without a single area of focus. Icon packs are especially important here as the screen is covered in apps. If you use a lot of obscure apps that are unlikely to be themed in icon packs, alternative icons and filters for unthemed icons matter.
Minimalist home screens
At the other end of the home screen spectrum lie screens with as little on them as possible. Minimalist home screens are tidy, they are simple, they are here to stay out of the way and let you get what you need to do done. The default layout on most third-party launchers, and even some manufacturer launchers like Google Now Launcher, lean towards the minimalist side so as to not overwhelm new users.
Sometimes, their home screen will be completely clear, just a small array of apps in the dock and a wallpaper that's easy on the eyes. Maybe there's a single widget at the top with the time and weather, or maybe another row of apps above the dock, but that's it. While this makes it easier to find the few apps that are there, this does mean that you'll dip into your app drawer more often, so app drawer organization becomes much more important.
Folders allow many of us to cheat, giving us a clean look on the home screen while still offering more apps outside the app drawer. When theming a lean home screen, you have much more freedom with your wallpaper, as that will be the most prominent element on your screen.
The need for your icon pack to compliment the wallpaper is higher with these layouts because the few apps that are there stand out more. Widgets with clear backgrounds play especially well with minimalist layouts, but a bright, custom widget can also serve as a bit of a statement.