Depending on how you build, it'll completely change what your team is capable of and its playstyle. For example, a Wind team with Titania makes a Burst build, but a Wind team with Gaia and Seth makes a strong Endurance build. A single pick is the difference between two very different builds. And by definition, you'll be picking a character you don't have and haven't had a chance to "experiment" with. In short, you can only experiment with what you have, and for most new players that won't be much; you have to rely on other sources of info for long-term planning.