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The argument is that everyone knows the next few years are going to be bleak, so it's better to hand over the clusterfuck to Labour sooner rather than later so the public associate the upcoming recession with them, rather than the Tories.
It has some merit, especially when you consider how well the Tories stuck the fault of the global economic crash of 2008 squarely at the feet of Gordon Brown (who'd, arguably, done more to ensure it wasn't as bad as it could've been than anyone else on on the planet).
Why would they call an election though? They've got a massive majority, so won't have to worry about one being forced on them, and if they called one they'd lose.
Why wouldn't they wait as long as they can?