Both the eyes almost see the same things throughout our life. It is not like we use one eye more than other that it should become weaker. What causes requirement for different power of lenses for both eyes? Shouldn’t they be same?
Because we have two hands, two feet, two ears, etc., we are tempted to think that we are perfectly symmetrical. But if you compare the details of your two hands, for example, you will find that the blood vessels are in somewhat different positions, the lines in your palms are different, and the fingerprints of corresponding fingers are different. The same is true of your eyes: they differ from each other. These kinds of differences are due to the fact that genetic instructions relating to symmetry during embryo development are not exact; they are approximate.
Probably if you measure the size of the biceps on each arm, or the size of the forearm muscles, you'll find that they are slightly different. This is partially due to the fact that you use one arm and hand differently from the other: you are right-handed or left-handed. Similarly, people tend to be left-eyed or right-eyed, using the two eyes differently. And, people tend to sleep more on one side than the other. These are differences that emerge due to experiences and habits.
Those factors make it unlikely that the two eyes will ever be identical, and make it likely that eventually they will need different eyeglass prescriptions.