Do I need restaurant reservations in Italy?

For dinner at anywhere good, yes — and increasingly for lunch in the big cities. Small trattorias have twenty tables and locals who book them. A day or two ahead is usually enough; for a notable restaurant in Rome, Florence or Venice in season, make it a week or more. Booking by phone still works better than any app, and hotels will happily call for you, which also gets you a better table. If you haven't booked, eat at 7:30 when tables are free, or accept that the place with immediate availability at 9pm on a Saturday has it for a reason.