During the 20th century, many designers started to look past the complexity in the Art Nouveau and Arts and Crafts Movement. It was not until the young artist Lucian Bernhard was kicked out by his father that he realized design could be simple. Plakatstil, meaning poster style in German, was thus created. The idea of using the simplicity of shape and color was primarily used in advertising to make their company easy to understand and remember.
Lucian Bernhard, Ludwig Hohlwein, and Hans Rudi Erdt were the three most prominent designers during this movement. All of which had many different ways and methods to create these posters. Even today, we still use the style of Plakastil to create a simple, clean, and almost satisfying design.
Sources:
https://www.britannica.com/topic/Plakatstil