This is a good question. Here are some points to consider. Often times the barrack emperors are also called solider emperors, because these men were first soldiers, who later on became emperors through military might. These emperors lasted from 235-284. Most importantly, these emperors did not create stability in the Roman government. The theory of might makes right is never a good policy. In fact, Rome was teetering on destruction by 284 and it was Diocletian, who saved the day. For this reason, he is the most famous and more important of the barrack emperors.
His contributions are far and wide. I will only name two of them. First, he tried to stem the problem of hyperinflation. We even have his price edict, which gave prices a maximum ceiling. So, we can say that he helped a runaway economy. As a side note, this idea of inflation is also something that we in the modern world need to consider.
Second and more significantly, he divided the government into two parts – the East and the West. He did this, because at this point Roman Empire was enormous. He also created a new system of rule were there were two leaders in each half of the empire, so that there would be ease of succession, which was often problematic for the Romans.