Which being translated, means: "Those who dream during the day know many things that escape the people who only dream at night."
I think that what Poe is getting at, is that there is a certain quality to the daydreaming mind, which allows the brain to make connections which it would not otherwise make. An article on Wikipedia explains "Daydreaming is the stream of consciousness that detaches from current external tasks when attention drifts to a more personal and internal direction."
It goes on to summarise an article about the five potential functions of daydreaming, from the Canadian Journal of Experimental Psychology, by Benjamin Mooneyham and Jonathan Schooler, the first two of which are future thinking and creative thinking:
"Future thinking, also known as autobiographical thinking, serves as a way to speculate and anticipate future events. Though it's costly for current external activities performances, the benefit will be paid off later, since future thinking allows better plan and preparation of the future goals.
Creative thinking is another function of daydreaming associated with increased creativity. When tackling unsolved problems, the most productive incubation periods in terms of creative solutions are those in undemanding conditions. ... Thus, it's reasonable to hypothesise that daydreaming plays an important role in generating creative problem-solving process."
So daydreaming can allow us to gently integrate our thoughts about future events, and to be more creative in our thinking. I'm sure that many of us are familiar with the process of worrying away at a problem, only for the solution to present itself when we give up and turn to something else. Barbara O'Brien describes this process beautifully in her book about her experience of schizophrenia, Operators and Things:
"The unconscious... when it is presented with a problem, does more than search its files with lightning fingers. It appears to search and also to consider, evaluate, weigh. First, it must understand the problem. And this it can also do. It can grasp an intricate concept. The conscious mind broods over its problem, and the unconscious, listening to the brooding, grasps the problem.
It searches its files, evaluates, and sends up an answer. The answer is rejected by the conscious mind. The conscious mind broods on the reason for the rejection and the unconscious listens, understands, gets to work again with the new concept and comes up with another answer. Still not good enough? Why? The conscious mind broods again and the unconscious gets to work again, and works until it finds an answer acceptable to the conscious mind. The conscious mind stops brooding and celebrates and the unconscious rests. For the time being, the organism is out of danger."
It is as though daydreaming allows people to think on a different level, and come up with creative responses to what our rational, conscious minds are thinking about. So yes, daydreaming can be beneficial, because it allows us to use more of our brains, to think more creatively, to dream of possibilities which might be rejected by our conscious minds, at least at first.