welfare has 9 questions, social-welfare has 24 questions. They are basically the same, so it makes sense to make one the synonym of the other. I suggest keeping social-welfare as the main tag because it is a bit more informative (e.g. avoiding confusion with subsidies, a term that is used for government money supporting businesses).
Welfare is a type of government support for the citizens of that society.
And above that it reads (like a comment):
"Social welfare" redirects here. For conceptual models of societal well-being, see Social welfare function.