Grit aids in this grinding process. Grit is vital to birds in the wild because they will eat things that are difficult to digest. However, in captivity, a bird's diet is fairly controlled. We feed our pets seeds which are easily hulled or pellets which can be ground up easily by the bird's beak and digestive system. This controlled diet eliminates the real and purpose of grit.
Nature seems to tell birds to keep a little grit in their gizzard.
There is a danger, though. Birds on a deficient diet, in trying to find nutrients lacking in their food, will often overeat grit. Excessive grit can irritate or even obstruct the digestive system of the bird causing numerous problems. In summary, a small amount of grit offered to your bird once every few weeks should not cause any problems. On the other hand, leaving a dish of grit in the cage at all times may be inviting problems.