Past - The Central Park Reservoir was a working reservoir, providing clean water to NYC residents. This was a necessary part of life in the city, helping sustain it.
Present - The now decommissioned reservoir is used to hold water used for other bodies of water in the park. This is helpful to maintain the aesthetic of the park, but it isn't entirely necessary for the day-to-day life of New Yorkers.
Future - In the case of a drought or drought-like disaster, the water in the reservoir could be purified as a supply for New Yorkers. This makes it a positive aspect of the supply system.