If a loved one dies, it's reasonable to be saddened by the event.
What does being sad accomplish? The person is still dead.
It's reasonable that people are helped, hindered, or otherwise effected by their emotions, as human beings are emotional creatures capable of reason. I think I agree with what I presume the point of the previous poster who asked if emotions are irrational behavior by definition. I guess it depends on what you're talking about specifically.
To answer your question, imho, what sadness might accomplish wouldn't be for the dead. I don't want to step on anyone's spiritual toes here, but these feelings are a natural response to a loss. The sadness, or any other emotion, is for the person feeling it. Perhaps it's so that we remember a defining, meaningful, influence in our life.
I read somewhere that women take in 7 times the emotional data that men do. If it's true, it makes sense to me because someone has to deal with a being that hasn't been taught anything, and empathy seems an efficient means to an end. We're capable of reason, but it's our cultural bank of knowledge that allows us to be as reasonable as we are. There's a good distance between 1 + 1 = 2, and spoken, and written language. In the time being, we only have our instincts, and our caregiver/s, to help us survive long enough to reason our way through some things, and we're capable enough to take care of ourselves. Perceivable emotional state would be a good asset for an infant. Though it's communication that doesn't require reason by the individual feeling it, it seems a reasonable evolutionary strength. An infant may cry the moment it feels hungry, or an object it was focused on falls out of sight. We, hopefully, don't cry when we feel hungry, because we know how to solve it ourselves. We also don't cry, again hopefully, when we lose our keys, but when we lose something that nothing else could prepare us for, like a parent, or sibling, or child, we're not quite as capable of staying rational. Perhaps it's just familiarity.. Maybe if one had a dozen kids, and lost ten of them, the tenth one lost wouldn't result in as much sadness.
We're social creatures. Even as adults, there are times when we rely on others. I imagine there is benefit there also for a perceivable sad state. We are capable of what we are capable of. Once we hit that stress threshhold, perhaps instead of failing, we invoke sympathy for assistance.
More specifically then "sadness", there are obvious benefits for fear, or regret, or shame, to a point. These emotions can keep you alive. They can severely limit you, too. With our reason, we can overcome any need for those feelings.
I'd like to pose a couple questions.. Would you think emotionless reason would be an advancement in evolution for human beings specifically? Wouldn't something like cancer, or intelligence on a computer, be the most reasonable, and efficient, form of life? If so, what would the point of that sort of life be? How much ambition would we have without emotion, and again, how valueable would that sort of life be?
Please excuse the rambling.. I started out thinking I was going to respond with about three sentences.