I'm a beginner, so why does the noise matter?

[Battou]

As much as I like your explanation the end result is that the picture looks grainy.

With some exceptions, grain is not nearly as...uniform...as niose. I wish I had an example for you but I have never had an issue with noise on my only digital camera. Also grain can be lessened during the shot as well as hidden with the texture of the subject at times where as noise can not, I have several shots taken with 800 speed film with little to no grain visable.

Conversely, CS3's noise reduction fails epically where grain is concerned, but it seems to work on noise.

 

[Ron Evers]

Back to the original question:

If you are shooting wildlife, the sound of the flipping mirror can spook the subject, limiting your opportunity for a second shot.

My S5 handles like a dSLR without the mirror & consequent noise, even the zoom is nearly silent. The other day I posted pictures of a mouse on our patio & if the camera had made any noise that little guy would have been long gone.

 

[Big Mike]

Also grain can be lessened during the shot as well as hidden with the texture of the subject at times where as noise can not

Sure it can. Digital noise usually shows up (is more visible) in solid colors, usually the darker ones. It is also a lot more prevalent when a shot is underexposed and is then pulled up in post. When shooting at high ISO (or maybe all the time) you can reduce the amount of digital noise by slighting overexposing or 'Shooting to the Right'.