This park is compared to Central Park in New York City. It is massive and one can wander the park all day and still not see everything in the park. The day we visited, there was a memorial in the park for a bombing that had happened at the Train Station, Atocha, in Madrid in 2004. Although we were in the park we did not see the memorial. We stopped at the park after taking the metro to Plaza del Sol to do some shopping.
So many people were in the park. People were picnicking, spending time with friends, spending time with family or just enjoying the park. As previously mentioned most people in Spain own their own apartments in the city. There are not as many houses as there are here in the United States. So going to the park is normal as they do not have as much living space in their apartments.
I would definitely want to come back to Madrid and spend more time in the park and maybe rent a boat and go to the glass castle in the park.