Present in fish in multiple arrangements, often multiple bones, often both on the dorsal and ventral sides. All other vertebrates seem to have them, too (at least in some fashion). Early mammals, starting with therapsids, seem to have undergone a fusion of the separate bones that there used to be (monotremes like platypuses and echidnas still retain an earlier multi-bone structure).
The biggest purpose is that in conjunction with the clavical it gives grounding to the arm and, more specifically, acts as the rotator cup of the arm.
It also helps to deal with the fact that we're ovals. How do you attach linear muscles to a round object? Break up the distance.
Secondly, how do you deal with a joint that needs to articulate back, but also down? As in, imagine if we didn't have a scapula and our shoulder muscles simply attached to our spine. Pulling back (like opening a door) and pulling down (like doing a pull-up) wouldn't be as distinguishable. The arm would just go back (but not down).
It's important to think about what the scapula tells us about our evolutionary history. Why don't we have a clavical set up like on our chest on our back? I.e., why not a joint stabilizing bone, but attach all our muscles to the middle (sternum or spine)? Well, because we used to run with our arms, because we used to climb and both those require some serious power and specificity.
Interesting to think about...
Well, anyway. We're a visual species. Here's some eye candy.