As a photographer, I can see an event or create a scene and capture it. Artists all go through the same process. But to bring it to light, the artists must have the skill to realize the work. The photographer must know how the image will be recorded in the camera, they must know the tones, contrast, color, or gradation of shadows before snapping the image. For black & white, a bit more knowledge is required so that the photographer can perceive in advance how the image will look in shades of grey, or capturing light which is so critical in black & white photography. Go up a skill level and the use of Photoshop or old school film developers allows even more control to bring the vision to reality.
The web is pretty amazing for finding solutions to almost any problem you might encounter (assuming you can get past all the SPAM sites trying to game Google or homeopathic websites which just seem to clog the web with disinformation). But, how often do you just randomly happen across a solution for something that you didn’t even know existed until you read it; especially if it isn’t a topic you follow regularly. Just as an example, here is a Complete Guide to Shooting Farming and Agriculture. I am sure most people wouldn’t think there were special requirements for photographing Bessy the Cow. One of the better ways of coming across more obscure information is to find blog posts in which the author has taken the time to scour the web for you and puts the results into list form. You then benefit from someone else having done all the dirty work.
Short answer: Because your photos will turn out better. Long answer: It’s a bit complicated, but here goes. First, lets talk about digital cameras. Behind the lens is a contraption called a CCD (Charged Coupled Device) that acts as an image sensor. It acts just like a piece of film. Basically, light hits it, and it records the information and transfers it to the the memory card in the camera as a picture. Simple so far. Now, I want you to think of all the information that the CCD records as a deck of cards. And lets use a photo’s brightness for this analogy. If the deck of cards is on the table, each card is a picture with the bottom card being a really dark version, the top card is a really light version, and the 50 cards in the middle are varying degrees of light to dark. OK, with me so far?