Absolutely nothing!
So why do we have to keep coming up with them? We’re writers, not cartographers. A synopsis is little more than a crude map leading from point A to Point B. No topography to give the journey depth. No color to distinguish the terrain. Just a “See Spot. See Spot run.” line from start to finish. A story is voice. A story is character. A story is conflict and tension, character arc and resolution. Where do you get that on a black and white map?
Can you tell I hate having to write a synopsis? I hate it only slightly less than I hate writing a query letter, but at least I understand the reason for a query letter. It’s not like agents are on the prowl looking for writers. In a perfect world, sure, that would be the case. But the world is anything but perfect.
I think it’s an ego thing. Imagine telling a new mother and father to describe their kid in five words or less. A writer can spend months on a single story, years on a novel. S/he has agonized over every twist and turn of the plot, every fumbling step of the characters and it all gets reduced to several pages of desiccated description. How is that helpful? If you walk into the auto showroom to look at that spiffy new roadster and all you can see of it is half of the right, front wheel, what does that tell you about the car? Where are the sleek lines, the bright colors, the growl of the engine, the feel of those fine leather seats? A synopsis is a little bit of chrome and a whole lot of flat black rubber. Are you gonna buy the car on the basis of that?
One could argue that it’s time constraints. I’d say that argument fails. Between five pages of dry synopsis and five pages of manuscript, I’d rather read the five manuscript pages. At least you get a sense of what the writer can do.