When the mind is one-pointed, it is said to be like a magnifying glass which is used to concentrate the sun's rays. Using a magnifying glass, a concentration of energy occurs which can even ignite an object in its path. Again, the mind can be compared to water which is released from a great height, such as a mountain. If the water has no channel it dissipates, but if a pipe is used to channel the water it flows down in a torrent, sometimes so strong as to sweep all obstacles, such as branches and trees, from its path. Yet again, the calm mind can be compared to still, limpid water, which is completely free of ripples, perfectly smooth. If one were to look in the water one would clearly see a true reflection of one's features. In the same way, the calm mind sees things undistortedly.
To put it even clearer, when water is still and calm, any dust or impurities in the water tend to sink into a sediment, leaving the water above clean and clear. Anything in the water, such as fish, snails, rocks and so on, is readily visible. Similarly, in calming the mind by practicing concentration (samādhi), there is a further benefit to be derived, apart from making the mind unperturbed and unmolested by defilements, and that is the arising of wisdom.
Usually, with a restless mind, whatever we look at we see unclearly. It is like trying to look at an object while it is swinging back and forth. No matter how hard we look we won't be able see it clearly. In fact, the more closely we try to inspect the object, the more blurred it becomes. If we want to see that object more clearly we must hold it as still as possible.
Our minds are like this. We are always having to deal with arising sensations but usually our minds are not calm. When the mind is not calm it is as if its object was being blown about in front of us by a strong wind. More refined things, such as particular problems we may want to resolve, are even harder to see clearly if the mind has no samādhi.
So we calm the mind. Calming the mind is like holding that object firmly and still, so that we can examine it as closely as we wish. Samādhi is a pre-requisite for wisdom. It is said, samāhito yathābhūtaṁ pajānāti: When the mind is firm and calm, wisdom functions clearly and we can see the truth.
However, simply having samādhi doesn't mean that one will automatically develop wisdom. If one doesn't know how to rightly use samādhi, one may simply settle for the calmed mind. Or one may think, "Oh, when I've calmed the mind maybe I'll be able to develop some psychic powers." One gets a desire for mental powers, such as divine sight, divine hearing and so on. These are all side-products of a concentrated mind. Samādhi which is practiced simply for its own sake is called samatha practice. It flies off in the direction of the refined absorption states (jhāna) and psychic powers. These are all fruits of mind-power.