There are no single reason why convention should be observed but a for an example, there is a convention that the Crown should appoint the leader of the party with majority of seat in the House of Commons who then is entitled to become the next Prime Minister. There are also conventions that the Parliament must be summoned at least once a year by the Crown. If that was not too happen then there would not be any annual Finance Act and the government would be only able to function by raising illegal taxation. Then by contrast, if the Crown broke the convention that the royal assent would not necessarily follow and the future of the monarchy would be at risk..
Conventions regulate the practice of Cabinet government and the law of the constitution makes only nonessential reference to the offices of the Prime Minister and Cabinet in the Ministerial and Other Salaries Act 1975 these exist by the virtue of the convention. Their powers are also determined by the convention rule as follows; the Prime Minister decides the national policy in discussion with a Cabinet. The Prime Minister calls Cabinet meetings and adopts national agenda.
Conventions regulate the work of Parliament and its rules and its conducts which are enforced by the Speaker and which can be incorporate in either House. The rules of parliamentary conduct are found in conventions are that the House of Lords should give way to House of Commons. Any financial measures introduced by the House of Commons should not be altered by the House of Lords. Conventions can be incorporated into statutes for an example as The Statute Westminster 1931.
Dicey took the view that the conventions as rules of ‘constitutional morality’, Dicey stated that it has had provide a ‘moral framework within which government ministers or the Monarch should exercise there non-justicable powers’. Hence, in domestic content, the rule that a government which loose confidence of the House of Commons must resign may be seen as an obvious requirement. Constitutional convention using Jennings' test three questions must be asked firstly, "What are the precedents?", secondly, "Did the actors in the precedents believe that they were bound by a rule?", and thirdly, "Is there a reason for the rule...”. Some of the conventions most important rules where in the case of Re Amendment of the Constitution of Canada where the Canadian Supreme Court’s took the view that ‘while they are not laws some conventions may be more important than laws’. The courts recognise and did enforce a convention which was stated in the case of Manuel v A-G (1983) and also in the case of Madzimbamuto v Lardner-Burke (1967). The courts recognise, but did not enforce conventions in the case of A-G v Jonathan Cape (1976)
In general it may be said that conventions differ from rules of law which are not enforceable but in practices conventions do regulate the residual legal powers of the Crown and the holders of the public office. Conventions are also giving a degree of flexibility to the constitution and in particular, they allow it to develop without alternating and develop by means of no existing legal rule. The most important reason for obeying conventions is to ensure that the machinery of government should function smoothly.
The Law of the Constitution (1885)
The Law of the constitution (5th edn), 1959