Maybe it don't have advantages, but I think not every manager has got the same chances when it's only implemented at new players.
And no, you don't keep all of the players of your starting roster, but most of the times the worse players will be sold, so the good players stay on the new teams and the old teams only get a chance to buy the worse players.. Which wil not happen because "old" teams don't buy worse players..
When I run into an invisible spider web, I instantly become a kung-fu master.