In an ideal situation you would use the aperture you want to use, the speed you want to use, and the ISO you want to use. So what is an ideal situation? Let’s say you want to photograph a vase on a table outside on a sunny day. The vase isn’t going to move, so you can use a slow shutter speed. There’s plenty of light, so you can use pretty much any aperture you want.
Let’s say you want a shallow depth of field, which means that the distance front to back that is sharp will be small. Everything outside of that depth of sharp focus will be blurry and out of focus – perfect for what you want. And with all that daylight you can use the lowest ISO, which is a good idea because the lower the ISO the less digital noise there will be in the image.
Now let’s suppose you want to photograph the vase being smashed with a hammer. Now you need to use a fast shutter speed. And it’s probably a good idea to have a bigger depth of field to catch all the shards of pottery that are flying all over the place, some further forward and some flying further back. So you have stopped down the aperture and increased the shutter speed – and suddenly you have hit the buffers.
Daylight is fading now and there isn’t going to be enough light entering the camera. So you increase the ISO, which makes the sensor react to less light, and put up with the increase in noise.
At the last moment you can’t find the hammer so you go back to taking photos of the vase, and to make it more interesting you put another object, an antique inkwell in front of the vase and you put flowers in the vase. Now you need to increase the depth of field to get everything in focus, and that means stopping down the lens to a smaller aperture.
But that means less light getting into the camera and you can either use a slower shutter speed to give more time for light to reach the sensor in the camera – or you can increase the ISO to make the camera more sensitive to light (and, noise).
A breeze springs up and now you have to use a faster shutter speed to freeze the movement. So now you really only have one option, which is to increase the ISO.
And that is how photographers have to work, thinking about what will work best for what they want to do.