Why do people in the Spanish-speaking world often call Chile’s capital city Santiago de Chile?

Santiago has coasts on both the Atlantic and Pacific Oceans.
Santiago has coasts on both the Atlantic and Pacific Oceans.

The National University of San Marcos is in Santiago.
The National University of San Marcos is in Santiago.

A city in Spain also has Santiago in its name.

To differentiate between the two cities, the Spanish-speaking world often refers to Chile's capital city as Santiago de Chile to specifically indicate that it is the city located in Chile. This is done to avoid confusion between the two locations.