If for no other reason but to tighten their grip on their own national minorities. The Basques and Catalans would have a viable case for independence if they could be EU members and most Western nations are founded on the principle of self-determination. Should Spain continue on its present course of denying self-determination to the Basque Country and Catalonia, they will reject the basic values that the EU is founded upon.

I'm not necessarily saying this as an advocate of Basque or Catalan independence. That is a question for the Basque and Catalan people. But a modern, moral, Western country would at least allow a binding referendum, as the UK has done in Scotland and as Canada did in Quebec. If Catalonia has a referendum and they choose to stay part of Spain, that is their choice. But the Spanish government refuses to grant them that choice.