What made socialism in 19th century Europe unique and worth supporting?
First of all, it is not clear that socialism was ever worth supporting. It is not clear that it would ever have worked or ever have been a good idea. Since your question presupposes that it was, however, we will assume that it was for the purposes of this answer.
If socialism was worth supporting, it was because it offered a more just and equitable system than was in effect in those days. In that time, Europe was industrializing rapidly and conditions for workers were very poor. They were treated badly and given very low pay.
In this context, socialism was unique because it provided a justification for putting the interests of the workers ahead of those of the rich. It would have been worth supporting (we can argue) because it would have made the lives of the mass of people less harsh and because it would have allowed them to get more of the wealth created by their labor.