The first and last perfect landscape was the Garden of Eden, created and defiled within the first few days of Genesis. Landscape Designers have been striving for this sort of perfection in their creations ever since. (But preferably with a bit more staying power.) They bring plants from all over the world together to replicate nature in the most unnatural ways. Plants from dry deserts, dripping jungles, dense forests, sparse tundra, lowlands and mountains come together and are expected to play nicely. It does not always work. That is what happens when you mess with the forbidden fruit.
Drawing a design and making it work are two different things. Plants are so much more than circles on paper. They are alive, dynamic and none too happy about what happened back in Eden. They can sometimes cooperate to function as we want them to, but only if they get what they want. Tropical plants cannot replicate a jungle in Alaska because they cannot take the cold. Cacti can not replicate a desert in Brazil because they rot with too much rain. Landscape Designers know what plants can survive (and hopefully thrive) in particular climates.
They also know how to place particular plants around buildings and other features that are already present in the landscape. Some plants can take the shade on the north side of a house. If the house has two stories, this shady area is a bit larger. Skyscrapers have huge shadows that can affect plants blocks away. No problem. Landscape Designers know what to do. And it doesn’t entail blowing up any skyscrapers either. Although they may be tempted.
Landscape Designers know what trees are good shade trees, but also what shade trees will be proportionate to a particular area. They know which trees are deciduous to let warming sunlight through in winter, and what trees are evergreen to obscure unwanted views. Vines can cover graffiti without damaging the wall. Shrubs can muffle the sound of traffic without taking over the neighborhood. Flowers can add subtle color without making the front yard look like a garage sale of 80's fashions. That is what Landscape Designers do.