Pressure times flow is
power.
Aircraft systems are high
power systems that do a huge amount of work moving controls, extending and
retracting flaps and so on.
To keep system weight
down they go for high pressure systems, at the limit that seals and pipes can
handle without blowing.
At this pressure limit
they still need a high flow to provide enough power for the job.
And please note that the
fluid is going round and round, out and back all the time.
The limit here is that if
you try and save weight by using less volume of fluid then fluid velocities
will increase, producing more heat and energy loss, even possibly fluid cavitation and mechanical damage.