Why do people in Barcelona speak both Catalan and Spanish?

In Barcelona, people speak both Catalan and Spanish because Catalan is the region's historical and co-official language, while Spanish (Castilian) is the official language of Spain and is widely spoken throughout the country. Catalan is a Romance language distinct from Spanish, though closely related to Occitan. It is spoken by millions in Catalonia, Valencia, and the Balearic Islands, as well as in parts of France and Andorra. While Catalan has a rich literary history, its use declined after the 15th century before experiencing a revival in the mid-19th century. Today, Catalan is taught in schools and used in media within Catalonia, coexisting with Spanish.1234