To be sure, the semi-auto modes (A and S) give control over the aperture and shutter but not over the exposure. The amount of light reaching the sensor will be the same in any of the modes, the same exposure.
There is a great myth about Manual, that it will give us "better" exposures, when in reality it will give the same exposure as the other modes since it uses the same light meter. The same settings are available in all the other modes as well.
Manual gives you better exposures, especially in difficult light because you know what you want exposed correctly.
ie.
that ball player with the cap, in the sun, will expose the scene to the person's face is dark due to the shadow made by the hat, your can choose to expose for the persons eyes, instead of the scene averaged together.
You have a back lit portrait, you choose to expose for the person, and blow out the background, instead of completely under exposing the subject, but exposing the background.
Another example, your dog, or kid are playing in the back yard, you have trees, and they are running in and out of shadow, if you use shutter priority, the exposure levels could vary a lot, if you use Manual, the exposure will be the same for all the shots.
Your kids playing in a little league game, they have white jerseys, the opponents are in black, the shift could fool the camera's meter.
Sure you can compensate for these things with exposure compensation, and metering modes, but sometimes it's just better to use manual, and there is really no reason not to when shooting in conditions with unchanging light.
