All of the photos below were taken in one day at Surrey Street Market in Croydon. The day was warm but the light kept changing with the clouds moving across the sun and then clearing again. The first six shots were all taken at ISO 100 and the second set of six at ISO 400.
The higher ISO have a greater noise and grain when enlarged, but sometimes it's necessary to capture a shot that wouldn't be possible in a low ISO. This might be a fast moving subject in low light or a concert where you can't use flash and need to up the ISO to be able to use faster shutter speeds. For modelling or product photography I can definitely see why grain and noise would be an issue. If the shot is going to be used in any kind of advertising likewise. However sometimes it is more important to capture the shot than to worry about any background noise.