The tax system, a continuity from antiquity, gave the medieval Roman state a huge military advantage.

The tax system, a continuity from antiquity, gave the medieval Roman state a huge military advantage. “East Roman military administration remained, until the 12th century, far in advance of that of its nearest neighbours…”

“The continued centralization of fiscal structures and consequent control over resources in materials and men (both politically and economically) gave the Byzantine government an advantage which none of its foes enjoyed. As long as the central government maintained its grip on the tax-extracting (or resource- extracting) machinery of the state, it was able to direct resources according to the requirements of defence or offence, and in accordance with the overall interests of the empire, to the best effect, in theory if not always in practice.” This is why the Romans could bounce back from defeats, the tax system gave it resources to form new armies and fight on, quite different from how a feudal western king raised his armies. The fact the Romans maintained salaried armies was decisive in their survival.

Nothing is perfect though, there was a downside. A poor emperor could misallocate these resources - which made taxation less justified and more resented. “Efficacious use of resources in men and matériel depended, of course, on those in authority recognizing where the priorities lay and not having to fight for their point of view to be implemented. That this was not always the case the  political history of the Byzantine empire all too frequently shows.” 

But overall, it worked well - giving the Emperors unique resources to deal their many challenges.

Warfare, State And Society In The Byzantine World 565-1204 by John Haldon