Training experience does provide us with a great place to start. No one would want to experience first hand why it is important to not hold your breath or to equalize your ears. This is where learning from others through classes and additional training comes in handy.
Practice also provides us with experience beyond what can be learned in a classroom. It allows us to take what we have learned in the classroom and use it to reinforce our learning in a controlled environment. Practice is especially important when it comes to emergency procedures. When an emergency arises, we fall back on what we have learned and practiced, so be sure to practice these skills often.
Diving provides the ultimate experience that is very difficult, if not impossible, to get while receiving instruction. It provides a real life application for all the things we have learned. It also allows us to become familiar with what diving is actually like. However, remember that quality and variety is more important than quantity. A diver who has done 1,000 dives all on shallow tropical reefs in relatively calm water is going to be less experienced than a person who has logged 200 dives in a variety of environments and conditions.