My understanding of the Westminster system is as follows:
- In a general election, the voters elect MPs to one or both houses of Parliament.
- The head of state appoints a Prime Minister who holds the confidence of a majority of the lower house of Parliament.
- The PM is required to retain the confidence of Parliament, and particularly must be able to pass supply bills and other basic legislation.
- 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.
- 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?