How so? Well, the theory is that if you give all voters a chance to pick a single party's candidate, then they're somehow invested in that candidate and will be more likely to vote for them come election time. In other words: the votes lost due to ill will against the outgoing Receipt Offender could be recouped by goodwill towards the incoming People's Choice.
Sure, we won't know for certain whether it will work like that until the next election. And the effect may not help Labour candidates so much, given the unpopularity of the government. But it's a straw I doubt they'll fail to clutch at.