to me the product development process is like a living organism. the goal of a living organism is to make its best efforts to live, continue life. similarly, product development must have a goal and the goal is to develop a product that solves a problem for the user. 

but a distinct characteristic of a living organism is that they can react to stimuli, the stimuli can be danger,  heat, cold, touch, anything. on similar vein, product development cannot be a static process, it must be able to respond to stimuli just that the stimuli, unlike in the case of a living organism, happens to be changes in requirements, competition, availability of new technologies etc

agile development gives us principles and tools that help us respond to change and to keep us to the goal of taking steps leading us to deliver a product that solves a user problem.