In the old testament, the hebrew word for prophet "navi'" also means "spokesman", and it refers to the general category of speaking on behalf of God. In the common english parlance, prophesy has a connotation of making predictions, but in the scriptures it has a much larger semantic range, including revealing some mystery or truth.
For example, telling a stranger that they are cheating on their husband. That is not a prediction of the future, but it's a revelation of a hidden thing that the speaker could not know just from his natural mind. So when Jesus told the Samaritan woman at the well her history of infidelity, she responded by saying "I see that you are a prophet" (John 4.19). He was not making any predictions to her when she said that (although he did make predictions after she said it).
These types of prophecies are immediately known to the hearer, whereas making a prediction requires waiting, and there is always some doubt as to whether the prophecy is not true yet. e.g. 1 Sam 19.23
"So Saul went to Naioth at Ramah. But the Spirit of God came even on him, and he walked along prophesying until he came to Naioth." it is much more likely that Saul was speaking hidden things revealed more generally rather than making specific predictions about the future. And the bible also speaks of processions of people annointed with the spirit walking down the street prophesying. Imagine a group of people telling the whole town secrets about your life as they walked down the street in a procession! And not only secrets about you, but also revealing secrets about the world more generally, of which the unknown future is just an example.
So I believe it is in this sense that both Moses and Paul are wishing for everyone to prophecy, e.g. to be able to speak the hidden things of God that come from the Spirit and so bring more revelation to the Church (in the case of Paul) or to the tribe of Israel (in the case of Moses). I don't think there is a specific benefit to making predictions about the future, except perhaps in the counsels of kings, but tautologically, there is a real benefit even in ordinary families to speak truths that were previously hidden.