Answer:
They built those kind of palaces because they wanted to showcase their power and prestige. Es Escorial in particular was built by Philiph II, one of the most important kings in Spanish history, who presided over the rule of the Spanish Empire when it was in expansion.
These kind of palaces could also instill fear on political opponents, and this is another reason why practically every European powerful monarch built a palace like El Escorial in their respective countries.