ISO's purpose is to help the photographer amplify light when your environment have insufficient amount of light to properly expose your photo.
Our day consists of morning, noon and night. During which time, light varies gradually. While on a bright sunny day, you can shoot at ISO 100, as the day progress, you will start to see that you would have to raise your ISO. Hence having the camera's sensor amplify the lack of light it's getting to properly expose your photo.
If you want the best image quality, always use the base (native) ISO of your camera. Base (native) ISO is the ISO range that doesn't involve software for example, Nikon has options in its camera like Lo-1 or Hi-1 which means that when you choose those ISO settings, its software generated or in other words, it "simulates" that ISO value thru software in-camera. So if your camera has a base ISO of 100, that’s your best foundation for the best image quality.
Not all sensors are made the same. With that being said, not all sensors can handle that same ISO value the same way. For example, an APS-C sensor's ISO 1,000 is totally different from a a Full Frame sensor's ISO 1,000. Full Frame sensors give better results in handling high ISO and have less grain or noise in a photo than other sensors.