My definition of Machine Learning in simple words:
Machine Learning is a way to let machine/computer/silicon learn from experience/existing-data in order to give a more educated guess regarding the future/unknown.
However the definition may not be exactly the same as the ones you find on a typical machine learning textbook. And here’s a bunch of more formal textbook definitions from Jason Brownlee’s wonderful post What is Machine Learning: A Tour of Authoritative Definitions and a Handy One-Liner You Can Use.
Why Machine Learning?
The intuition is that if a machine learning method is evolved by experience, then the method is more flexible (i.e., less problem specific), thus can be applied more generally.