It can reduce one's clawback on your current tax year. If the higher income earner is the one splitting the pension, providing they split enough, it could reduce their clawback. Alternatively, if the lower income spouse's total income goes up too much, it could mean theirs gets clawed back. 

What the optimizer does is, after all the calculations, it deems the correct amount to minimize your combined taxes payable. I recommend running the optimizer as the last thing you do before filing.