Intellectual Property is a pretty big area, but I'm going to pretty much stick to pictures here. Copyright, for our purposes, is the legal machinery around ownership of intellectual property (yes, yes, there's lots of other stuff, patents and trademarks and trade secrets blah blah blah, I don't care). A photograph is copyrighted, and it is that legal machinery that provides the maker of the photograph with a meaningful idea of "ownership" of that photograph as an abstract image, not merely as a physical artifact.
Intellectual property in and of itself is arguably a pretty odd idea, and it is certainly just a social construct. The idea of property of any kind is a social construct, after all, and ideas like "land ownership" are virtually incomprehensible to some societies which remain with us to this very day. Be that as it may, we have a notion of intellectual property in general, and legal machinery exists very specifically in the western world to create a clear notion of ownership of a picture.
Another traditional and particularly clear example of a copyrightable piece of intellectual property is the novel. The particular sequence of words that makes up a novel, the sequence itself, is an ownable thing, a piece of property. There are a couple of dimensions to this:
Until recently it was pretty hard to actually make a copy of a novel. The means of production was constrained. An individual could copy a novel, but it was quite difficult. Even the dullest person could see that there was some value in a physical copy of a novel.
Orthogonally, even the dullest person can see that creating a novel, devising that unique sequence of words, is real labor. Not just any fool can be taught to do it even acceptably well, it takes a degree of talent. It takes many many hours of mental effort. Furthermore, the maker's stamp is more or less indelible: while many people can write a novel, only Agatha Christie could have written Murder on the Orient Express.
There are, therefore, at least two dimensions in which people perceived value in a copy of a novel, value in the book they held in their hand.
Contrast this with, say, a typical wedding photographer in this current era:
Making copies of the photographs from the wedding costs nothing. It's all digital, and I can have my local drugstore print out copies of digital photographs for pennies a picture. I can post a digital picture to a dozen places on the internet to share with my friends for nothing at all. Digital images have perceived value approximately equal to 0 cents.
Taking wedding photographs is certainly a craft, like plumbing. Not just any fool can do it, but almost any fool can be taught to do it. Furthermore, wedding photographs tend to look pretty much the same. Bob's wedding photography may be distinctly different from Alice's wedding photography, but Ralph's, Susan's, and Albert's wedding photographs look a heck of a lot like Bob's. Finally, it's pretty easy. There's some hours of labor involved, before, during, and after. It's work, but it's a lot more like plumbing than it's like defusing bombs or writing novels.
Photography loses in both dimensions: it's digital, and thus trivially copied; and it's easy.
The perceived value of wedding photographs has always been quite different from the perceived value of a novel. People have always suspected that commercial photography is really work for hire. They are paying to have something recorded, and are vaguely surprised and irritated to find that the photographer mysteriously retains ownership of the pictures. A plumber does not retain ownership of the pipes he installs in my home, after all.
This perception is increasing, and will continue to. More and more, people are simply ignoring the copyright held by the photographers, and posting wedding photos, school photos, portraits, and other commercial work on their facebook pages. The sole-practitioner photographer can rail about it and send angry letters, but not much else. The commercial photographer is free to "educate" the customer about copyright, and to try to explain why it is that they "own" the pictures. The commercial photographer is also free to buzz down to the beach and attempt to hold back the tide with a stick. Insofar as anyone bothers to think about it, there simply does not seem to be a supportable rationale for "ownership" of the intellectual property inhering in wedding photographs. There's the law, but the law appears to be an ass.
Digital media has dropped the value of a song, a picture, a movie, to very close to zero. Stock photographs are virtually free. I expect to be able to watch all the movies I want for a modest monthly fee. Ditto music. A song is worth -- at most -- 99 cents, and to many people that's much too high. It is simply not reasonable to expect that wedding pictures, engagement photos, baby portraits, are going to retain high values. They are not. When you shoot these things, the perception today is largely that you are performing skilled labor for a period of time, and that alone is the basis on which you should be compensated.
The legal machinery of copyright isn't going anywhere any time soon, but it is increasingly becoming irrelevant and ignorable. Speed limits aren't going anywhere any time soon either, but it's not like anyone follows them.
This sucks for people who want to make money taking pictures. I have some ideas on that point, coming up.