The administration of the Dominate (aka Later Roman Empire) was different in many ways from the Principate (aka Early Roman Empire).
In most cases it is rather clear why the various reforms had to be made. For example, breaking up the provinces into smaller units was part of the overall bureacratization which was itself an expedient way of extracting more revenue from a bedraggled land and population to maintain larger armies (and a larger bureacracy to boot).
However, I don't quite understand why the civil and military parts of the administration were separated.