El Clasico is a match that captures the world's attention, for a huge span of reasons.
It's a commercial behemoth, with not just merchandise but sponsorships and in-game advertising slots banking on it for notice. It's a huge draw to new potential fanbases, with one exerting dominance and influence over the other, showing the world they are the ones to follow.
Lest we forget, it's also somewhat of an important sporting event: often a title-decider even when it's mid-season, at other times a game that can dictate where a trophy may end up—just like at the start of this season when Real Madrid lifted the Supercopa in Spain by vanquishing Barcelona 5-1 on aggregate.