Many Italian restaurants prefer customers who make a reservation, even if the reservation is made just hours in advance, by phone or in person. It's a commitment that is valued and respected. Reservations are required at the most popular dining establishments. A few places require at least a week in advance, but very few, and it depends on the time of year.
If you make a reservation and change your mind for whatever reason, you must be courteous and cancel your reservation, hopefully in a timely manner.
It's never good to show up too late for a reserved table, no matter what your reasons are for being tardy. Always best to show up five minutes early and allow for travel errors.
Most tourists do not reserve at restaurants. They don't speak the language and are afraid to make a phone call and deal with English not spoken well or at all. Most tourists will go from restaurant to restaurant when they're hungry and check out the menus posted outside. The more popular places (always with limited tables) will be booked sooner, so those who did not reserve will be turned away. Those who are turned away will repeat the cycle until they a find an affordable place that has a table available. On certain nights, some people search for a long time.