I would think crying and any outward sign of emotion is something that evolved in humans to augment communicating feelings in a social species. I would imagine long before humans could effectively communicate with words, they communicated with facial expressions, body language/body displays coupled with making sounds.
I believe evolution provides the explanation for many of these enigmas people want to attribute to a god having created the world and designed us. Humans WERE designed...in a manner of speaking. They were designed by their environment operating on a malleable heredity. Humans...and other species...survive if their gene pool produces individuals who pass on their genes. And perhaps the human species survived better than other hominid species because of their various abilities..and one ability that aided the survival of the gene pool might have been their ability to communicate...including communicating sadness. '
The other option is that human's crying, ,if not an beneficial adaptation in its own right, might be an inadvertent outcome from some other trait that WAS a benefit to survival. IOW, it may be neutral to survival and only arose because it was an accidental trait coupled with some other beneficial trait.
Certainly other species cry...at least they 'cry out.' And one can easily see how an individual...say, caught by a predator might benefit the rest of the 'herd' by crying out because then the others might flee or fight...depending on their nature. And the 'tears' humans shed might be an inadvertent added tendency that is of no benefit.
Tearing up seems it would be a benefit in some cases, because it cleanses the eyes, and, so maybe it kicks in, for some reason, when the person is sad.