As far as the way our country is run, the separation of powers is a vitally important doctrine of law. This covers the three branches of power within the country and these are the executive (the government); the legislature (Parliament); and the judiciary (the courts system).
It is a well known fact that the more inter-linked these powers are, the more trouble is likely to arise. In other words, each of the three branches need to be separated from each other as much as possible. The reason for this is so that each crucial branch of power needs to be left to their own devices and not dictated to from the other branches. As you can imagine, this is especially important where the executive's influence over the judiciary is concerned.
If you research other countries, it is interesting to see what the extent of separation of powers is in operation in their state, and then make a comparison with the type of regime that exists. For example, a truly oppressive country would see no true separation in the three branches of power.
Interestingly, The United Kingdom does not have such an identifiable separation of powers as can be found in The United States; even though they have pretty much inherited our own legal system. However, there are numerous conventions in place that mean that there are certain aspects of the separation of powers of this country that mean that there is always a level of inter-linking at times.