Because they think they can take it over and break it from the Labour Party!
Their analysis is that organised left grouping within Unite, the United Left is weak and they can take it over. As that grouping controls most of the Unite NEC they think they will take over the NEC. Once they have the NEC then they will break Unite from the Labour Party.
Of course this is the sort of machine politics that the SP has successfully practiced in PCS (take over Left Unity, LU takes over the union etc). Therefore they believe that Unite will be just PCS writ large.
Also they are pinning their hopes on all the public sector workers in Unite (council workers, NHS and former PCS) being put into one group. Such a hope is mentioned in a PCS bulletin and by their speakers. Of course they would hope to dominate this public sector group (PCS would be the biggest component of such a set up) and so be bigger fish in a bigger pond than at present in PCS. This group would be their power base within Unite and no doubt a source of full time officer jobs for their comrades.
Of course all the above is mechanical (everything just follows from one thing to another; take over the United left, then the NEC etc.) and does not involve winning the members over but SP would believe that if you win the superstructure of the union and the organised left, then the members will follow – or at least they will not get in the way.
We have a different conception of politics to the SP. Ours involves winning members and activists to ideas, creating a rank and file movement that really does control the leadership; not as a means to winning places on committees. Also as said the SP conception is mechanical. For example if they did succeed in winning the United Left in Unite they seem to think they could just take it over intact. But it is highly likely that the UL would split and a new ‘left’ would be created. Certainly the Labour Party and full time officials would not stand idly by and let SP operate unchallenged.
So what would the members get out of all this? Well no doubt a SP comrade would argue that Unite under their control or if they had a significant voice within it, would be a radical union in the same way as PCS; that breaking from the Labour Party is a vital necessity for the working class.
We have our doubts as to the former. Whilst PCS does take strike action it does not- as you would assume a radical union would- openly challenge other unions, appeal direct over the heads of other union leaders to the members in those unions, argue a radical change in the political system (the union’s demand for PR maybe part of such a change but in itself is not that radical change) and for a radical democratisation of the economy (saying that austerity should end is not the same thing as saying who should own what).
The SP have made no substantive real case as to why a transfer to Unite would be better for members but, as we argue above, the proposed link up with Unite is not really for the members.