Consider the following design points:
The more items powered by a hydraulic system, the more likely it is to fail. There are more pipes and couplings to break and lose system pressure.
So critical systems (flight controls, brakes) need to be powered by multiple systems, of which at lest one will do little else.
Each hydraulic system should have at least two sources of pressure. The engine driven pump and something else. The something else's could be an auxiliary pump (electrical or air driven), a power transfer unit (hydraulic pump driven by another hydraulic system) or the RAT. Obviously secondary pressure sources aren't any help if the oil has gone.
The loss of a single system will mean you've lost some services and other critical services will probably be downgraded (e.g. Less effective flight controls)
A RAT is there as the final source for critical services. It may also be able to supply critical electrical power.