Rome by this point was mostly known as being a religious and cultural center, and the Italian city-states didn't really see themselves as heirs to the Romans, so why was capturing the city and making it the capital so crucial for their new kingdom?
Rome by this point
Sorry, can you clarify what event and what time you are asking about?
In short, because they did see themselves as heirs to Rome. Rome was seen as a unifying idea. It was powerful, the Empire was built by Italians and it was wealthy. Taking Rome for their capital would help create a unifying idea and strengthen their claim to the old Empire. It's why the HRE, Byzantine Empire, Napoleon, the Ottomans and so many others wanted it. Look at the Italian national anthem and all of their references to Rome. When Austria collapsed, the Italian Prime Minister called it a Roman idea. On two occasions since Rome fell, have Italians tried to establish a Roman Republic. The Fascists were quick to claim Rome too.