(Not to mention all the other images of police violence).
Being able to say "I have given them the referendum, they are not taking it" vs giving the other party such a great narrative "That's why we want independence. We are more civilized than them. They are aggressive animals." changes something.
Their narrative would just change slightly, to not getting a "real" or "fair" referendum, one that they could win. And they would not be wrong.
Yes, sending the riot police was a very, very bad move in terms of defusing a conflict. Literally all they had to do is let them run their big show, and expectedly declare the result void. At most, prosecute the politicians that unilaterally proclaimed independence in the parliament based on that puppet referendum.
However, the political party in power at the time, Partido Popular, knew that they would win some (right/fascist) votes everywhere else in Spain by exceeding force in Catalonia, in a kind of "scorched earth" move. They historically got almost no votes represented out of Catalonia anyway.
The alternative was to send police to fire rubber bullets, with somebody losing an eye (https://www.spainenglish.com/2019/10/01/eye-rubber-bullet-ca...), against people that wanted to... vote.
Rubber bullets happen to be a weapon invented by the British to use during The Troubles (https://en.wikipedia.org/wiki/Rubber_bullet). But even the British, the inventors, stopped used them. In fact, the Catalan local police were also banned from using them and were shot in Catalonia only because it came from the non-Catalan police (https://www.independent.co.uk/news/world/europe/catalonia-pr...).
(Not to mention all the other images of police violence).
Being able to say "I have given them the referendum, they are not taking it" vs giving the other party such a great narrative "That's why we want independence. We are more civilized than them. They are aggressive animals." changes something.