Those are the spatial dimensions.
Before we can solve "real" problems in our 3-dimensional world we need to understand the basic properties and equations involved. It is much easier - and better for learning any topic - to start with simpler examples that then develop into the more complex situations.
Just as in mathematics we start with addition before progressing to multiplication, and we need to understand algebra and geometry before we can do trigonometry or move on to calculus. In Physics we start with the simplest system descriptions - one dimensional - and then develop the equations into the two- and three-dimensional applications.