The simplification provided by a good abstraction layer allows for easy reuse by distilling a useful concept or design pattern so that situations, where it may be accurately applied, can be quickly recognized.
Just composing lower-level elements into a construct doesn't count as an abstraction layer unless it shields users from its underlying complexity.
Each layer of the model encapsulates and addresses a different part of the needs of digital communications, thereby reducing the complexity of the associated engineering solutions.
The operating system then performs the actual transformation needed to read and write the stream of bytes to the device.
The specific device commands for a plotter are different from the device commands for a CRT monitor, but the graphics library hides the implementation and device-dependent details by providing an abstract interface which provides a set of primitives that are generally useful for drawing graphical objects.