At a simplistic level its about checks and balances.
The HS can control the judiciary system if it looks as if its going to get out of control. Exactly the same the judicial system prevents the government getting carried away with power
The result has been that England and later Britain has been very stable - not to many revolutions
The monarchy, Parliament, and the courts of law all vie with each other for power but all end up being counterbalances to the other part of the (triumvirate not sure if that the right word?)