Speculation rarely brings knowledge in the short-term. But it helps to distinguish true knowledge from hypothesis. Most of the time we just accept what is socially correct to accept. By speculating about what the world may be like we arrive at the conclusion that most of the things we thought we knew are in fact just speculations.
Another advantage is that, if we take a specific speculation or hypothesis and develop it to the maximum extent we can, we may be able to arrive at some unexpected predictions. We may be able to turn it into a viable hypothesis, something that can be tested, confronted, with experience.
In ancient Greece, almost three thousand years ago and for a period of a few centuries, people started speculating about what made the world the place it is. There have been other explanations, not speculative but dogmatic, they were made by people with authority or that searched for authority and that purportedly explained how the world was like, why we were here, what we should be attempting, etc.
Speculating about the world may seem a futile task. Why should we speak about what we ignore. The answer is simple: by researching different possibilities we might find out ways of testing them. For instance, in the debate between the atomists and animists there was, at the time, no way to decide between the two. Leucippus and Democritus speculated that the world was entirely made of atoms, tiny pieces that moved in a mechanical way. All over the universe these atoms would make up the stars, the animals and so on. According to Leucippus "Nothing happens at random, but everything from reason and by necessity." On the other hand we had another speculation, articulated for instance by Plato in the Timaeus, according to which matter was the mix between absolute chaos and absolute order. According to this view matter would behave partially erratically, although it had also, at the same time, some degree of order given to it by the universal laws or archetypes. But in this perspective it would make no sense to try to predict exactly how a rock would fall, because there was simply no precise way for it to fall. Precise measurements would be considered futile when applied to a chaotic nature. Precision could only be found in the areas of pure thought like mathematics and geometry.
So in this debate we already find two distinct hypothesis: either everything in the world is understandable and behaves by necessity, or there is an element of chance or irrationality. But if the world is understandable then its understanding must involve very complex explanations, because what we see is very diverse. I mean the components of the world, which behave always in the same way must be able to build things like water, air, fire, living tissue, the moon, the sun, hearts and bones, thoughts and ideas. It is all part of the same universe. So I can the building blocks of the universe be able to construct such diverse things. And besides, we divide the water and the air and there seems to be no limit for that division. So it appears that these building blocks must be very tiny and that the explanation of the universe must be very complex and, possibly, be vastly more complex than what we can understand.
All this can be understood as a consequence from accepting the atomic hypothesis and by developing it to its full consequences. The problem with the ancient thinkers was that they become one-sided. They largely preferred Plato's and Aristotle's physics, which were also hypothetical, and disregard the atomic hypothesis. This was largely because of the ethical and political consequences of the atomic hypothesis: if true knowledge was so eloigned from our current capabilities then most of our theories and mythologies would be wrong. The atomic hypothesis would displace the common explanations of religion and philosophy, dispelling them as mere myths or imaginative stories. Shattering all our confidence in them. Showing them as what they were: mere speculations too.
As we can easily see, the greatest advantage of considering various speculations is not so much in the short-term advancement of knowledge, its main advantage is in removing the dogmatic way of thinking, making us face our true ignorance in the face of the vast world and its many possible interpretations.