I hope this signing indicates that Turner will be given the number 3 shirt next year, with Martin being signed as backup should things not work out, or if an 'old head' is needed on that flank (particularly in defense).
The back end of this season Turner looked good, scored and set up a few trys. His build is perfectly suited to the centre spot, and maybe a full off season training in that position will do him the world of good.
Martin will be able to pass on a few things to him, particularly positioning in defense (which I think is his main weakness).
People need to realise it's only a 1 year deal, meaning the club are waiting to see if Turner matures, or waiting for a top class centre to become available. He's a stop gap (just like Phelps was) which will provide much needed backup should either centre get injured.