There's no absolute answer to this, as it's essentially a matter of opinion. However, different groups have given theories as to the purpose and necessity of the state and it's government which I will summarise a few of below:
Marxist perspective
Marx believed that the state existed solely to benefit the bourgeoisie, or the class of people whose main concern is to keep their economic supremacy over society.
The necessity of either would only depend on your position in society, as to people considered to be working class, it isn't beneficial.
Some consider Marx's theories outdated, but modern Marxism has adapted them to apply to current society.
Capitalist perspective
One definiton of 'state capitalism' is a state where the government controls the economy and acts essentially as a corporation.
This can include the abolition of services provided by the government, such as free healthcare and welfare systems. Businesses can then buy some of these nationalised services and run them for profit.
The necessity of this system is questionable, as it seems to serve business owners over 'the people'. However it can be argued that work undertaken by corporations is better, as they solely exist to make money, therefore it's in their interest to provide high quality services. If you can afford them.
Anarchist perspective
Contrary to popular belief, anarchy does not mean destruction. Although it has been known for people to associate themselves with anarchy while committing destructive acts, it can be defined as 'a society without a publicly enforced government'.
Anarchists argue that state and government negatively impact on an individual's autonomy, or freedom.
An example of this is the voting system, which offers several candidates to choose from, but these candidates can never truly represent your own feelings and opinions. Instead you are expected to pick the one that most closely resembles these.
These are just a few perspectives on state and government, but as you can see the definition and considered purpose and necessity differs hugely, depending on the theory you most associate with.
Some other theories to look into are feminist, pluralist and the work of Michel Foucault.
This TEDx talk gives some helpful insight into the division of power in government: