To some people (myself included), forming a union contract is an improvement in itself, even without any immediate changes. If conditions are already good for workers, unions can help keep them good.
Could you expand on why you feel this is the case? If conditions are "good," in my opinion, adding a union would likely add tension between employer / employee and add costs to both sides. I guess if nothing immediately changed, it would likely only add cost to the employees in the form of union dues, I'm not sure if a company typically pays for the pleasure of having a union.