Sadly the Spanish government has made the mistake of trying to repress the vote. Polls indicated, (before the attempt to repress by arresting 14 leaders of the Catalonia government, seizure of the ballots, and threatening 700 judges who offered to open their courtrooms to voters), that the population would vote it down. The highest court in the land has said the vote is not allowed under the Spanish constitution (in agreement with the federal attorney's office -- like the US Attorney General) but the Catalan leadership insists they will hold the referendum no matter what.
Two years ago a non-binding referendum on the same question was held and the measure was voted down. The difference between then and now is that this referendum is binding….meaning the government of Catalonia would be required to act upon it, declaring independence.
If you ask why they want to be independent there are two basic reasons: economic and cultural. Catalonia is the richest and most prosperous part of Spain. Thus, like anywhere else, a percentage of their economy supports areas outside their area. In other words, they have a negative tax balance with their tax dollars supporting others. In addition, they speak a different language and the Spanish government has, in the past, tried to restrict the use of that language (think French in Quebec and if the Canadian government suddenly tried to get the Quebec to stop speaking French). So they want to leave for economic and cultural reasons.
The European Union will not recognize an independent Catalonia. The US will not recognize an independent Catalonia. Spain (of course) will not recognize an independent Catalonia. In the end even if they voted for independence AND declared independence the only thing that would happen is that they would then enter into a civil war they would lose and end up ruining their own economy for years, if not decades, to come. Perhaps that is why the Spanish government's repressive measures actually made things worse. For if they had let the vote proceed, and if the polls were wrong and Catalonia voted for independence, and if the Catalan leaders declared independence, and if Spain recognized Catalan independence, and if the the world also recognized it, well, then Catalonia would enter the world as the 187th independent nation. But if any one point (other than, perhaps Spain recognizing Catalonia independence) things don't go right for the independence movement, then Catalonia would be plunged into an economic no-mans land at best and a civil war at the worst.
Thus, on the question of Catalan independence, if I were a Catalan and allowed to vote, I'd vote "no," even if I were for it.
AJ