When planning a train trip through Europe, you have two choices - using a rail pass of some kind or getting point-to-point tickets. There are pros and cons to each option.
Point-to-point tickets (also known as city-to-city tickets) are good for traveling shorter distances, such as between two cities or towns within one country. They can also be a good value if you plan to travel only to a few places.
The drawbacks to buying point-to-point tickets as you travel include waiting in ticket lines at train stations, dealing with language barriers as you purchase tickets along the way, and possibly paying higher prices for tickets on same-day travel. You can avoid these issues by getting your city-to-city tickets through ItaliaRail before you leave home, but if you add spontaneous train trips to your itinerary in the middle of your trip you may end up spending more on tickets than you would have on a rail pass.
Rail passes tend to be a better value if your itinerary is primarily covering long distances - especially between multiple countries - or on high-speed trains for which tickets are more expensive. They also give travelers the ability to be more spontaneous by not requiring an exact itinerary beforehand.
The drawbacks of rail passes include potentially paying a higher price per train trip than it would have cost you to buy individual tickets (if you knew your exact itinerary in advance), and the need to buy seat reservations or pay a supplemental fee for high-speed trains. Because of the latter, even with a rail pass you may be required to wait in those long ticket lines anyway.