Digital cameras need certain levels of light to function and create usable images. The sensor needs a certain amount of light to reach it for it to be able to work properly.
This can be controlled by shutter speed – shortening the shutter speed to let in less light and lengthening it to expose the sensor to more light. However, in some instances, when faster shutter speeds or longer exposures are needed, digital cameras can effectively lower or raise the sensor’s sensitivity.
Yet there is a downside to raising the IOS value.
Rather than raising the actual sensitivity of the photosites, it amplifies the electrical current they generate. This means that the noise is also amplified, leading to blotchy grains on the final image.
To try and keep image quality to the highest standard you should only, adjust IOS when absolutely necessary.