Mentioning my main interest for travel to Spain (Flamenco) in the cover letter

I would say that it is not needed, but it will not harm you if you write it down.

For other types of visa (for example family reunion, or business) it could be necessary to establish the reason of the travel (who are you going to meet, if you have a reason to meet them...).

But the reason to chose a particular touristic destination is usually very personal and as such they will not help the officer reviewing your application in determining if you are a legitimate applicant or not (as long as it does not seem incoherent¹).

Seeing flamenco, seeing this or that monument, this or that tradition, that beach with good waves... each person's interests are very personal and difficult to gauge. For example, to me it is uncomprehensible that people may travel thousands of kilometers to see a soccer match, but that happens, and if I were a border officer I would not deny visas to see a soccer match just because the reason seems crazy to me².

So, stating that your reason to go there is Flamenco gives no idea to the officer about how likely you are to respect the visa, and it just "colours" the relevant point which is that this is a touristic visa. What will decide if you are given it is everything else in the application (how the officer assesses your risk of overstay or that you are making a dishonest application). If they believe that you are truthful and just want to do some tourism, they could not care less about what exactly you want to do there.

But, if you want to, write it. Just write it as a short statement so it does not distract the officer from reviewing the rest of your document.

¹For example, probably it would raise some flags if you said that you wanted to go to Spain to see the Big Ben.

²But I would deny them if the match the applicant wants to see is not scheduled to happen at the time of the travel.