It's the dimension which defines movement. It's a measurement of place as compared to movement or change. If place did not change, nor movement did not occur, there would be no need for time. However, even a stationary object has movement on a quantum scale, therefore time exists.
We experience time in a linear fashion because it is most cohesive toward reproductive success. That statement is hypothetical, as I believe it is very difficult for a human mind to comprehend such a thing for entirely that purpose. Therefore, it's also completely unscientific, but fits all other teleological explanations of the human condition. That being said, it also makes sense in the context of the adaptation of intensive child-rearing in humans - parents that saw time in the same way as they saw length, width and height may not raise their children in a progressive fashion. There is no inherent reason for this other than evolutionary impulse.