Splits up Spain that much more. Recall that it was once one of the world's great powers, contending with Britain and France for colonial power back in the day.

Right now the ultra lefty voters in Barcelona are cancelled out by being attached to the rest of Spain. If they were made independent they would very likely immediately be admitted to the EU where they would add to the globalist voting bloc. They could also be used as another convenient back door into europe for human trafficking.

Also, as I mentioned elsewhere, they don't just want an ultra-left city to secede, I'd actually support that much in the same manner as excising a cancer. They also want an enormous rural area which is mostly conservative and anti-communist.