In a Westminster system, why would the PM voluntarily dissolve a majority government?

Question

My understanding of the Westminster system is as follows:

1. In a general election, the voters elect MPs to one or both houses of Parliament.
2. The head of state appoints a Prime Minister who holds the confidence of a majority of the lower house of Parliament.
3. The PM is required to retain the confidence of Parliament, and particularly must be able to pass supply bills and other basic legislation.
4. If the PM loses the confidence of Parliament, or just feels like it, they advise the head of state to dissolve Parliament, and a new election is held, restarting the process.
5. Sometimes something goes badly wrong and people get upset about it.

What I don't understand is why a PM would voluntarily dissolve Parliament under a majority government. If the PM's party formed a coalition which is now in danger of falling apart, it would make sense to try to time the election for maximum political effect. Similarly, if the PM's party formed a minority government with some kind of confidence-and-supply arrangement, that would also have the potential to motivate an early election.

But if there is no coalition, and the PM's party holds an absolute majority of seats, what benefit does the PM gain by trying to hold an election right now? They already have enough votes in Parliament to pass whatever legislation they like, so getting a "mandate" from the voters seems useless. The party itself might fall apart, but that is a relatively rare event while elections are quite common.

Many Westminster-system Parliaments have maximum terms, and the UK itself now has "fixed" terms that can still be cancelled by a no-confidence motion, but historically this has not been the case (admittedly, 17th century Britain is hardly an example of the modern Westminster system).

Is the danger of running out of time and having a possibly inopportune election really the only motivating factor? If so, why has (for example) the Parliament of Canada's four year term never been allowed to expire? I would expect PMs with absolute control of Parliament to try to keep that control for as long as possible, and occasionally to be unable to come up with a "good" time to hold an election, until time runs out and the election holds itself. Why doesn't this happen often or at all?

Answer 1

One motivation is to choose a time for the next election that is favourable to the party in power.

If the current term is nearing completion and opinion polls are currently favourable but expected to decline, it makes sense to secure power now for the next five years (or other term).

Examples

Exclusive: Tories call for snap general election as polls show Theresa May would nearly quadruple Commons majority

Telegraph. October 2016.

Wikipedia has a list of snap elections which includes examples of majority governments seeking a strengthened mandate.

Answer 2

Doubtless you will recall that, in the 17th Century, Oliver Cromwell continually extended the Parliamentary term every time it came near to expiry, by having it vote a succession of extensions, such that the Parliament elected in the 1640s was still sitting in 1660.

It suffered so much from deaths in battle, and by natural causes, and also from the actions of Cromwell himself, that it became known to history as the Rump Parliament, because only a small 'rump' of the original MPs survived to carry on the business of Government.

In World War 2, elections were not held. The parliament's term was voted to be extended until the cessation of hostilities, and no election was held until the end of the fighting in Europe.

Governments today, whilst the Fixed Term Parliaments Act survives (so not for much longer!), can't choose when to hold an election, because a 2/3rds super-majority is needed to call an early election, and the Government no longer has even a simple majority in Parliament. The Act was introduced to protect David Cameron from being stabbed in the back by the Liberals, in the coalition of 2010, but it makes no sense in a parliament where there is no coalition government (i.e. 99 percent of the past 350 years).