Since Leonardo was an architect as well as an inventor, he decided to combine those skills to create something beneficial to others as well. You see, in the renaissance age, toilets weren't quite a thing yet. In short, people would dump their waste in the streets. This was an extremely unsanitary method and overall it was most likely very disgusting. The ideal city was Leonardo's solution to this problem. It was basically a plumbing system and sanitary solutions to other problems. Unfortunately, this solution was not used or put into reality until the proper toilet was invented in 1597.