This is a great question. Not all big scale projects benefit the local economy. However, the zoo in Pittsburgh has been a blessing to the economy and the city. Here are some statistics that might help you.
First, the zoo is a big tourist attraction and it draw more than all the local professional sports team. Over 130 million people visit each year! As you know this is important, because Pittsburgh needs jobs, since steel is no longer big business in the area.
Second, apart from the economic benefit, the zoo was able to teach children and even adults about the importance of the environment and things like the conservation of nature.
Finally, the zoo has been a blessing to the community as an outlet for friendships. In short, the city is better because of the zoo.