Chances are excellent that you won't
need one to stay at any of Spain's youth-hostels. You
may be charged a little more for not having the card they accept but that's the thing; some require one card and others require another. If the youth-hostel (called "albergue juvenile" in Spanish) is an official youth-hostel of the
International Youth Hostel Federation (a.k.a. "Hostelling International" - the logo is the house and pine tree in the blue triangle) you'll get a small discount for being a member. And you can always buy these cards at your first stop. Thing is, you may not use it enough to make your money back in discounts. This happened to us when we were traveling around Europe about 15 years ago.
Again, it just depends on the youth-hostel and not every one will accept all cards as they might not be in that particular association.
Good luck! Saludos, MadridMan
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