The theory is that Labour will hold on to more mayoralties on second preferences. Of course that only works if they're in the top two...
More broadly there's a belief that this will disadvantage Reform - Labour are doing all sort of interesting things as part of Operation Stop Nigel - but if Reform get enough votes, it might not matter.
Someone should really tell them that we already had a referendum on the supplementary vote, we said no, and they can't just keep sneaking it into sub-national elections.