|Well, I'll try and contribute, too...|
For #1, I think photography can be extremely effective, if not one of the most effective ways to communicate thoughts, feelings, etc. The biggest reason is that a photograph can get it's point across in any language or any culture. Think of some of the most famous photos in the world -- the US marines raising the American flag on Io Jima for example: That's a picture of hard work and triumph.
How about pictures of war, poverty or famine? Don't you think these photos stir strong emotions in people? Photos can bring out good feelings, too. Landscapes are known for their peaceful and tranquil nature. Nudes can bring out sensuous or erotic feelings.
Question 2: The biggest impact today is the digital photography revolution. This has enabled people to share many more visual images at such a rapid rate. Families can be on vacation on one side of the world and send pictures back to friends... Businesess can communicate with visual descriptions instantaneously.
Question 3: I'm not a historian, so I don't know the exact answer, but, if I recall, the "camera obscura" was invended in the greek or roman days. It was a dark room that had a pin hole in one wall. The view, or scene outside the room was projected through this hole and showed up as an inverted image on the wall oposite the pin hole. This principle is identical to a "pin hole camera" and very similar to a modern film camera.
Now, a means to record this upside down image didn't come around for many years (centuries). There are theories that early "life-like" paintings were painted under these camera obscura projections in the 1400 and 1500's. It wasn't until the very late 1700's or early 1800's that "film emulsion" was invented -- and this was really plates of silver, which then evolved to silver on top of glass, and this evolved to the film we know today.
Question 4: Clearly one doesn't have to look far to see how photographs have documented events -- look at all the printed matter around us. For me, the photographs that have the most impact are ones that have an irreproducible quality -- ones that show a specific moment in time that can't be reproduced. Frequently images of sorrow and suffering fall in this category -- pictures of the american indians walking the trail of tears/pictures of the nazi concentration camps... Of course there are pictures of great triumph -- athletes setting records/children accomplishing something for the first time, etc.
Question 5 is a little too vague for a straight answer... the very nature of being human, and belonging to an intelligent species, is communicating with these beings. As humans, we have many ways to communicate, and most of them are directly related to one of our 5 senses (touch, taste, smell, sound, and sight). Images are one of the stronger methods of communicating visually with others. From scratching lines in the dirt with a stick to advanced digital photgraphy used to perform internal surgery, we rely on images to relate with others. Why does photography matter? In a sentence: it matters because we see.