score:3
I would say yes to some extent.
They want to know why you wish to travel to each of those countries. So if you says "due to my love of Flamenco...." then this an interest in the local culture which presents a valid reason for travel there as a tourist.
But they also want to be sure that you will return home when you are finished. So you could also say something like "...I hope to improve my Flamenco guitar skills and impress my friends when I return home."
But you should be clear and to the point. No need to mention everything.
Here is an example: https://flightitineraryforvisa.com/cover-letter-for-schengen-visa/
Edit: To answer your other questions.
By everything, I mean that you don't have to tell them every detail of your trip and your life story in a cover letter. No need to tell them of every single place you will visit, or your guitar lessons back home. But tell them how long you will spend in Spain and Netherlands and/or provide flight/train/bus tickets etc.
As long as you are not being unreasonable, how long you stay is a matter of how much funds you have. A longer stay means you must have more funds available. But 15 or 25 days is not going to make much difference.
Upvote:1
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.