Client states were very useful as buffer states for the Romans. This also meant that they had to have their own military forces. Preferably, the Romans did not intervene whenever client states were attacked, unless it became clear that they were unable to deal with the threat.

A good (maybe slightly old, but still useful) book about this is Luttwak's The Grand Strategy of the Roman Empire.

Source: Luttwak, E. (2016). The Grand Strategy of the Roman Empire: From the First Century CE to the Third. JHU Press.