Key functions of political parties in a democracy

• They help voters to make sense of politics. 
• They provide candidates for general, local and European elections. 
• They enable people to participate in politics. 
• In the UK they are the basis for choosing a Government and opposition. 
• They provide a focus for particular policies that can be put into place by the largest group in parliament.

The third point is essentially a factor in a liberal democracy  at election time, although membership of parties is decreasing significantly.
In regards to the fourth point, the party with the majority of seats forms government, and the second largest forms the opposition.

In a liberal democracy, parties are about choice and…