Latency and audience
Madrid puts your server 5-15 ms from the average Spanish ISP and 25-40 ms from Paris, Milan or Lisbon. It is the natural DC when your audience is Spain, Portugal or Spanish-speaking LATAM: for that traffic, no central-European or US DC beats it on perceived latency. Typical fits: SEO ES, e-commerce and SaaS aimed at the Iberian market.