Illustrations have been a form of expressions right from ancient times even when writing had not evolved as a form of expression. But even after that, when you look into history, even until photography as a concept has not yet been devised, people only relied on illustrations for depiction or replication of any noteworthy visual.
Back in those days, there was even more pressure on an illustrator as they were the only source of expression of a visual concept. The imagery has to be as close to real as possible, as that was the only possible way before photography entered the scene.
Even today, illustrations form one of the most important aspects of newspapers and magazine articles. Though now the pressures on an illustrator are lesser with respect to their images being as close to real as possible. They are given a lot more freedom in expressing themselves and to use their creativity. However now there is more pressure with respect to creating an original concept. So the importance has now shifted from their technical skills as an illustrator to their creativity and to the newness in any visual that they come up with.
This also gave rise to the unique styles of illustrations that have come up in recent past. The modern forms of illustration are quite different from the traditional ones, simply because the traditional ones involved more hard work in terms of technical skills.
The modern day illustrations can now get the exact same effect thanks to modern technology. And so the focus has now shifted to the concept and its originality.
Types of illustrations can broadly be classified into two main groups.